Pump unit

ABSTRACT

The present invention provides a pump unit that includes at least one hydraulic pump with inlet and outlet ports formed therein, a pump case for accommodating the at least one hydraulic pump, the pump case having an opening through which the at least one hydraulic pump is insertable into the pump case, and a center section connected to the pump case in such a manner as to close the opening of the pump case. The center section forms a pair of inlet/outlet passages having first ends respectively communicating with the inlet and outlet ports of the at least one hydraulic pump and second ends opening to the outside of the center section through a pump case abutting surface of the center section, and a first charging passage having a first end through which working hydraulic fluid is fed into the center section and a second end opening to the outside of the center section through the pump case abutting surface of the center section. At least one of the pump case and the center section forms a communication passage for communication between the second ends of the pair of inlet/outlet passages and the second end of the first charging passage. A first hydraulic fluid feeding valve is also provided in the pump unit for allowing hydraulic fluid to flow from the first charging passage to the pair of inlet/outlet passages while preventing the reverse flow. The first hydraulic fluid feeding valve is installable through the pump case abutting surface of the center section or a center section abutting surface of the pump case.

BACKGROUND OF THE INVENTION BACKGROUND OF THE INVENTION

[0001] The present invention relates to a pump unit used for variouspurposes.

[0002] More particularly, a first aspect of the present inventionrelates to a pump unit that includes a hydraulic pump, a pump case foraccommodating the hydraulic pump and a center section connected to thepump case.

[0003] A hydraulic pump is used in various applications and inparticular as the hydraulic pump adapted for operation in associationwith an actuator driven through the hydraulic effect. In this case, thehydraulic pump is connected to the actuator via a pair of hydrauliclines, and the output flow rate of hydraulic fluid discharged from thehydraulic pump is varied to cause the pressure difference between thepair of hydraulic lines, thereby driving the actuator. When thehydraulic pump is thus connected to the actuator via the pair ofhydraulic lines so as to constitute a closed circuit, a chargingmechanism is generally required to feed pressurized hydraulic fluid tothe pair of hydraulic lines.

[0004] Specifically, the charging mechanism as required necessarilyincludes a charge line having a first end through which pressurizedhydraulic fluid is fed into the pair of hydraulic lines, and a secondend communicating with the pair of hydraulic lines, a checking valve forallowing the pressurized hydraulic fluid to flow from the charge line tothe pair of hydraulic lines, while preventing the reverse flow.

[0005] For the hydraulic pump with the charging mechanism, it isdesirable to reduce machining works for the reduction of themanufacturing cost, and/or improve an assembling efficiency ininstalling the check valve, or other works However, there have not beenmade effective proposals in view of those points.

[0006] The first aspect of the present invention has been thereforconceived in consideration of the prior arts. It is an object of thefirst aspect of the present invention to provide a pump unit with thecharging mechanism for feeding additional hydraulic fluid, which pump isused in association with the actuator driven through the hydrauliceffect, and is capable of lowering the manufacturing cost and improvingthe assembling efficiency.

[0007] A second aspect of the present invention relates to a pump unitwith first and second hydraulic pumps that is designed to be operated inassociation with an actuator driven through a hydraulic effect.

[0008] A hydraulic pump is used in various applications and inparticular as the hydraulic pump adapted for operation in associationwith an actuator driven through the hydraulic effect. The descriptionwill hereinafter be made for the pump unit by taking for example thecase where it includes the first and second hydraulic motors serving asthe actuators that respectively drive the right and left drive wheels.

[0009] For example, U.S. Pat. No. 4,920,733 discloses a vehicleincluding first and second hydraulic pumps respectively connected viafirst and second hydraulic lines to the first and second hydraulicmotors for driving the right and left drive wheels. In this vehicle, thefirst and second hydraulic motors respectively have outputs variable inresponse to the adjustment of the input/output flow rates of the firstand second hydraulic pumps, thereby controlling the rotational speed androtational direction of the right and left drive wheels.

[0010] The vehicle of the above arrangement has the first hydraulic pumpand the second hydraulic pump separately arranged from one another, theformer being operated in association with the first hydraulic motor, andthe latter being operated in association with the second hydraulicmotor. Such a separate arrangement of the hydraulic pumps involves atroublesome piping work between the first and second hydraulic pumps andthe first and second hydraulic motors, a troublesome assembling work ofthe pump unit, and pose various other problems.

[0011] The second aspect of the present invention has been thereforconceived in consideration of the above prior art. It is an object ofthe second aspect of the present invention to provide a pump unit withthe first and second hydraulic pumps that is capable of achieving thesimplification of the piping work between the actuator and the hydraulicpumps, and the assembling work of the pump unit.

[0012] A third aspect of the present invention relates to a pump unitused for a vehicle with first and second hydraulic motors respectivelyconnected to the right and left drive wheels, and includes first andsecond hydraulic pumps that are designed to be respectively operated inassociation with first and second hydraulic motors.

[0013] There are known arrangements of the above type described in, forexample, U.S. Pat. No. 4,920,733. According to this US patent, a vehiclewith first and second hydraulic motors respectively connected to theright and left drive wheels includes first and second hydraulic pumpsrespectively operable in association with the first and second hydraulicmotors. In this vehicle, the first and second hydraulic motorsrespectively have outputs variable in response to the adjustment of theinput/output flow rates of the first and second hydraulic pumps, therebycontrolling the rotational speed and rotational direction of the rightand left drive wheels.

[0014] The vehicle of the above arrangement has the first hydraulic pumpand the second hydraulic pump which are separately arranged from oneanother, the former being operated in association with the firsthydraulic motor, and the latter being operated in association with thesecond hydraulic motor. Such a separate arrangement of the hydraulicpumps poses various problems, such as troublesome mounting operation ofthe hydraulic pumps on the vehicle, and troublesome assemblingoperation.

[0015] The third aspect of the present invention has been thereforconceived in consideration of the above prior art. It is an object ofthe third aspect of the present invention to provide a pump unit usedfor the vehicle with the first and second hydraulic motors respectivelyconnected to the right and left drive wheels, and including first andsecond axial piston pumps of a variable displacement type that iscapable of having improved efficiencies in mounting the pump unit on thevehicle, and improved assembling efficiency.

[0016] A fourth aspect of the present invention relates to a pump unitwith first and second hydraulic pumps that are respectively connectedvia first and second hydraulic lines to first and second actuatorsdriven through a hydraulic effect.

[0017] A hydraulic pump is used in various applications and inparticular as the hydraulic pump adapted for operation in associationwith an actuator driven through the hydraulic effect. The descriptionwill hereinafter be made for the pump unit by taking for example thecase where it includes the first and second hydraulic motors serving asthe actuators that respectively drive the right and left drive wheels.

[0018] For example, U.S. Pat. No. 4,920,733 discloses a vehicleincluding first and second hydraulic pumps respectively connected viafirst and second hydraulic lines to the first and second hydraulicmotors for driving the right and left drive wheels. In this vehicle, thefirst and second hydraulic motors respectively have outputs variable inresponse to the adjustment of the input/output flow rates of the firstand second hydraulic pumps, thereby controlling the rotational speed androtational direction of the right and left drive wheels.

[0019] The vehicle disclosed in the above cited US patent has the firsthydraulic pump and the second hydraulic pump separately arranged fromone another, the former being operated in association with the firsthydraulic motor, and the latter being operated in association with thesecond hydraulic motor. Such a separate arrangement of the hydraulicpumps invites a complicated structure of a feeding passage for feedingworking hydraulic fluid from a reservoir tank to the first hydraulicline and the second hydraulic line, and poses various other problems.

[0020] The fourth aspect of the present invention has been thereforconceived in consideration of the above prior art. It is an object ofthe fourth aspect of the present invention to provide a pump unit withthe first and second hydraulic pumps that are respectively connected viathe first and second hydraulic lines to the first and second actuatorsdriven through the hydraulic effect, and that is capable of achieving asimplified structure of the feeding passage for feeding workinghydraulic fluid to the hydraulic lines extending between the actuatorsand the hydraulic pumps.

[0021] A fifth aspect of the present invention relates to a pump unitwith first and second hydraulic pumps that are respectively connectedvia first and second hydraulic lines to first and second actuatorsdriven through a hydraulic effect.

[0022] A hydraulic pump is used in various applications and inparticular as the hydraulic pump adapted for operation in associationwith an actuator driven through the hydraulic effect. The descriptionwill hereinafter be made for the pump unit by taking for example thecase where it includes the first and second hydraulic motors serving asthe actuators that respectively drive the right and left drive wheels.

[0023] For example, U.S. Pat. No. 4,920,733 discloses a vehicleincluding first and second hydraulic pumps respectively connected viafirst and second hydraulic lines to the first and second hydraulicmotors for driving the right and left drive wheels. In this vehicle, thefirst and second hydraulic motors respectively have outputs variable inresponse to the adjustment of the input/output flow rates of the firstand second hydraulic pumps, thereby controlling the rotational speed androtational direction of the right and left drive wheels.

[0024] The vehicle disclosed in the above cited US patent has the firsthydraulic pump and the second hydraulic pump separately arranged fromone another, the former being operated in association with the firsthydraulic motor, and the latter being operated in association with thesecond hydraulic motor. Such a separate arrangement of the hydraulicpumps invites a complicated structure of a feeding passage for feedingworking hydraulic fluid for an HST (hydrostatic transmission) from ahydraulic fluid tank to the first hydraulic line and the secondhydraulic line, and poses various other problems.

[0025] As a further disadvantage, the working hydraulic fluid betweenthe hydraulic pumps and the actuators may increase in temperature due tothe load from the outside. Such an increase in temperature of theworking hydraulic fluid may invite various problems such as lowering ofthe volumetric efficiency, or lowering of the axle revolution speed ifthe hydraulic motors are used as the actuators for driving the drivewheels of the vehicle, deteriorating of the durability. However, theabove-cited US patent does not teach any solutions to limit thetemperature of the working hydraulic fluid of the HST.

[0026] The fifth aspect of the present invention has been thereforconceived in consideration of the above prior art. It is an object ofthe fifth aspect of the present invention to provide a pump unit withthe first and second hydraulic pumps that are respectively connected viathe first and second hydraulic lines to the first and second actuatorsdriven through the hydraulic effect, and that is capable of effectivelylimiting the increase in temperature of the working hydraulic fluid tobe replenished to the hydraulic lines between the actuators and thehydraulic pumps.

SUMMARY OF THE INVENTION

[0027] According to the first aspect of the present invention, there isprovided a pump unit that includes at least one hydraulic pump withinlet and outlet ports formed therein, a pump case for accommodating theat least one hydraulic pump, the pump case having an opening throughwhich the at least one hydraulic pump is insertable into the pump case,and a center section connected to the pump case in such a manner as toclose the opening of the pump case. The center section forms a pair ofinlet/outlet passages having first ends respectively communicating withthe inlet and outlet ports of the at least one hydraulic pump and secondends opening to the outside of the center section through a pump caseabutting surface of the center section, and a first charging passagehaving a first end through which working hydraulic fluid is fed into thecenter section and a second end opening to the outside of the centersection through the pump case abutting surface of the center section. Atleast one of the pump case and the center section forms a communicationpassage for communication between the second ends of the pair ofinlet/outlet passages and the second end of the first charging passage.A first hydraulic fluid feeding valve is also provided in the pump unitfor allowing hydraulic fluid to flow from the first charging passage tothe pair of inlet/outlet passages while preventing the reverse flow. Thefirst hydraulic fluid feeding valve is installable through the pump caseabutting surface of the center section or a center section abuttingsurface of the pump case.

[0028] With the pump unit of the above arrangement, the workload forboring the center section can be reduced, and assembly efficiency inassembling the pump unit can be improved.

[0029] The pump case of the pump unit of the first aspect of the presentinvention is preferably adapted to reserve the hydraulic fluid.

[0030] The pump unit of the first aspect of the present inventionpreferably has the following arrangement. Specifically, the workinghydraulic fluid fed into the first charging passage through the firstend thereof is hydraulic fluid fed from a charge pump operativelyconnected to a driving shaft for driving the at least one hydraulicpump. The center section of the pump unit forms a second chargingpassage for communication between the inside of the pump case and thefirst charging passage; and the second charging passage preventing theflow of the hydraulic fluid from the first charging passage into thepump case, while allowing the hydraulic fluid stored within the pumpcase to flow into the first charging passage when negative pressure isgenerated in at least one of the pair of inlet/outlet passages,

[0031] The communication passage of the pump unit of the first aspect ofthe present invention preferably has a groove shape and formed in thecenter section abutting surface of the pump case. The pump case also hasthe center section abutting surface forming an escape groovecommunicating with the inside of the pump case for the escape of theleaked hydraulic fluid.

[0032] Further, the center section of the pump unit preferably forms abypass passage for communication between the pair of inlet/outletpassages; and the bypass passage provided with an open/close valve insuch a manner as to be operable from the outside for communication andcutoff of the hydraulic fluid between the pair of inlet/outlet passages.

[0033] According to the first aspect of the present invention, there isalso provided a pump unit that includes a first hydraulic pump withinlet and outlet ports formed therein, a second hydraulic pump withinlet and outlet ports formed therein disposed parallel to the firsthydraulic pump, and a pump case for accommodating the first hydraulicpump and the second hydraulic pump. The pump case has an opening throughwhich the first hydraulic pump and the second hydraulic pump areinsertable into the pump case. The center section is connected to thepump case in such a manner as to close the opening of the pump case. Thecenter section forms a first pair of inlet/outlet passages having firstends respectively communicating with the inlet and outlet ports of thefirst hydraulic pump and second ends opening to the outside of thecenter section through a pump case abutting surface of the centersection, a second pair of inlet/outlet passages having first endsrespectively communicating with the inlet and outlet ports of the secondhydraulic pump and second ends opening to the outside of the centersection through the pump case abutting surface of the center section,and a first charging passage having a first end through which workinghydraulic fluid is fed into the center section and a second end openingto the outside of the center section through the pump case abuttingsurface of the center section. At least one of the pump case and thecenter section forms a communication passage for communication thesecond ends of the first pair of inlet/outlet passages and the secondends of the pair of second inlet/outlet passages to the second end ofthe first charging passage. A first hydraulic fluid feeding valve isalso provided in the pump unit for allowing the flow of hydraulic fluidfrom the first charging passage to the pair of inlet/outlet passage andthe second pair of inlet/outlet passages. The first hydraulic fluidfeeding valve is installable through the pump case abutting surface ofthe center section or a center section abutting surface of the pumpcase.

[0034] According to the second aspect of the present invention, there isprovided a pump unit that includes a first hydraulic pump and a secondhydraulic pump respectively connected to first and second actuators viaa first pair of hydraulic lines and a second pair of hydraulic lines.The first hydraulic pump and the second hydraulic pump are accommodatedwithin a common housing with the former pump disposed parallel to thelatter pump. Both first and second hydraulic pumps are supported on acommon center section. The common center section forms a first pair ofinlet/outlet ports and a second pair of inlet/outlet ports. The firstpair of inlet/outlet ports respectively serve as connection ports forconnection with the first pair of inlet/outlet hydraulic lines, whilethe second pair of inlet/outlet ports respectively serve as connectionports for connection with the second pair of inlet/outlet hydrauliclines.

[0035] With the above arrangement, the conduit connection between thefirst and second actuators can be accomplished via the common centersection, thereby achieving an improved efficiency in piping work. Inaddition, the first and second hydraulic pumps are accommodated withinthe common housing, so that the first and second hydraulic pumps can beinstalled on an object such as a vehicle through a single mountingoperation.

[0036] The common center section of the pump unit of the second aspectof the present invention preferably forms a common charging passage forfeeding pressurized hydraulic fluid into the first pair of hydrauliclines and the second pair of hydraulic lines. With this arrangement, thenumber of charging lines can be reduced as compared with the arrangementwhere the first and second hydraulic pumps are separately installed,resulting in a lower manufacturing cost.

[0037] The first and second pair of inlet/outlet ports of the secondaspect of the present invention are preferably formed in the same sideof the common center section, thereby achieving an improved efficiencyin piping work between the first and second hydraulic motors.

[0038] The pump unit of the second aspect of the present inventionpreferably has the following arrangement. Specifically, the firsthydraulic pump and the second hydraulic pump respectively have pumpshafts connected together by a power transmission mechanism provided inthe common housing. The common housing includes a partition wall throughwhich the pump shafts of the first hydraulic pump and the secondhydraulic pump can extend. The partition wall divides the common housinginto a pump accommodation chamber and a power transmission mechanismaccommodation chamber. With this arrangement, a single powertransmission path is sufficient for the simultaneous rotation of thepump shafts of the first and second hydraulic pumps, resulting in asimplified structure of the power transmission mechanism for the powertransmission from the power source to the pump unit. The partition wallcan effectively prevent foreign matters such as iron powder generated inthe power transmission mechanism from intruding into the pumpaccommodation chamber.

[0039] The pump unit of the second aspect of the present inventionpreferably has the following arrangement. Specifically, the firsthydraulic pump and the second hydraulic pump are of an axial piston typethat include angularly adjustable swash plates of a cradle type,respectively having rear sides forming spherical convex surfaces. Thepartition wall forms guiding surfaces respectively sized and shaped toslidingly guide the spherical convex surfaces of the angularlyadjustable swash plates along the guiding surfaces. These surfaceformations can achieve lower manufacturing cost of the hydraulic pumps,and stabilized operation of the angularly adjustable swash plates.

[0040] According to the third aspect of the present invention, there isprovided a pump unit used for a vehicle with first and second hydraulicmotors respectively connected to the right and left drive wheels.Specifically, the pump unit includes a first hydraulic pump and a secondhydraulic pump, both being of a variable displacement type locatedparallel to one another within a common housing, and respectivelyoperable in association with the first and second hydraulic motors. Thefirst hydraulic pump and the second hydraulic pump respectively includea first pump shaft and a second pump shaft located parallel to oneanother within the common housing and operatively connected to oneanother via a power transmission mechanism. The first and second controlshafts are designed for controlling the input/output flow rates of thefirst and second hydraulic pumps. The first and second control shaftsrespectively extend away from one another along the vehicle widthdirection.

[0041] The pump unit of the above arrangement can achieve simplifiedmounting of the pump unit on the vehicle, and simplified structure forthe power transmission between the power source and the pump unit. Whenthe pump unit with the first and second control shafts extending awayfrom one another along the vehicle width direction is mounted on avehicle having push-pull control levers, the first and second controlshafts can be disposed parallel to the push-pull type control levers,thereby achieving a simplified link mechanism between the control shaftsand the control levers.

[0042] The first control shaft and the second control shaft of the thirdaspect of the present invention are preferably located substantially atthe same position with respect to the vehicle longitudinal direction.This arrangement can achieve a more simplified link mechanism betweenthe control shafts and the control levers.

[0043] The pump unit of the third aspect of the present inventionpreferably has the following arrangement. Specifically, the housingincludes a partition wall between the first and second hydraulic pumps,and the power transmission mechanism, through which the first pump shaftand the second pump shaft can extend. The partition wall divides thehousing into a hydraulic pump accommodation chamber and a powertransmission mechanism accommodation chamber, thereby effectivelypreventing any foreign matters such as iron powder generated in thepower transmission mechanism from adversely affecting the pumpperformance.

[0044] The pump unit of the third aspect of the present inventionpreferably has the following arrangement. Specifically, the first andsecond hydraulic pumps are of an axial piston type that respectivelyinclude angularly adjustable swash plates of a cradle type respectivelyhaving rear sides forming spherical convex surfaces. The partition wallforms guiding surfaces respectively sized and shaped to slidingly guidethe spherical convex surfaces of the angularly adjustable swash plates.With this arrangement, the hydraulic pumps can be manufactured at lowcost, and the angularly adjustable swash plates can be securelyoperated.

[0045] According to the fourth aspect of the present invention, there isprovided a pump unit for operation in association with first and secondactuators. The pump unit includes a first hydraulic pump and a secondhydraulic pump respectively connected to the first and second actuatorsvia a first pair of hydraulic lines and a second pair of hydrauliclines; a center section supporting the first hydraulic pump and thesecond hydraulic pump; a housing accommodating the first hydraulic pumpand the second hydraulic pump. The first hydraulic pump, the secondhydraulic pump, the first pair of hydraulic lines, the second pair ofhydraulic lines, the center section and the housing are integrallyconnected together to constitute a single unit. The pump unit alsoincludes a reservoir tank supportingly connected to the single unit forstoring hydraulic fluid to be replenished to the first pair of hydrauliclines and the second pair of hydraulic lines.

[0046] The pump unit of the above arrangement can improve an efficiencyin mounting the first and second hydraulic pumps on an object such as avehicle, and shortening the length of the conduit for replenishing thehydraulic fluid from the reservoir tank to the first pair of hydrauliclines and the second pair of hydraulic lines, thereby lowering themanufacturing cost, and improving an efficiency in replenishing thehydraulic fluid through the decrease of the resistance force between thehydraulic fluid and the conduit wall, and producing other desirableeffects

[0047] Preferably, the single unit of the pump unit of the fourth aspectof the present invention is designed so that the housing can serve as ahydraulic fluid tank, and the pump unit further includes a hydraulicfluid communication passage for providing a free fluid communicationbetween the reservoir tank and the housing. With this arrangement, thenumber of the conduits required between the first and second hydraulicpumps, and the first and second actuators can be reduced tosubstantially four conduits only, specifically the first pair ofhydraulic lines and the second pair of hydraulic lines. Thus, ascompared with the conventional arrangements, the pump unit of thisarrangement can achieve a lower manufacturing cost, an improvedassembling efficiency and an excellent workability in maintenance. Sincethe housing itself also serves as a hydraulic fluid tank, the reservoirtank can compactly be made.

[0048] The pump unit of the forth aspect of the present inventionpreferably has the following arrangement. Specifically, the centersection of the pump unit is a unitary member for supporting both firstand second hydraulic pumps. The center section forms a first pair ofhydraulic passages respectively having first ends communicating with thefirst hydraulic pump and second ends opening to the outside of thecenter section to form connection ports for connection with the firstpair of hydraulic lines, a second pair of hydraulic passagesrespectively having first ends communicating with the second hydraulicpump and second ends opening to the outside of the center section toform connection ports for connection with the second pair of hydrauliclines, and a charging passage having a first end opening to the outsideof the center section to form an inlet port for charging, serving as aninlet for the hydraulic fluid to be replenished and a second endcommunicating with the first pair of hydraulic passages and the secondpair of hydraulic passages via check valves. The charging passage isconnected to a pressure relief line communicating with the housing via arelief vale, and the inlet port for charging is connected to thereservoir tank via a hydraulic fluid replenishing passage.

[0049] The pump unit of the fourth aspect of the present invention alsopreferably has the following arrangement. Specifically, the centersection includes a first center section and a second center sectionrespectively supporting the first hydraulic pump and the secondhydraulic pump. The first center section forms a first pair of hydraulicpassages respectively having first ends communicating with the firsthydraulic pump and second ends opening to the outside of the firstcenter section to form connection ports for connection with the firstpair of hydraulic lines. The second center section forms a second pairof hydraulic passages respectively having first ends communicating withthe second hydraulic pump and second ends opening to the outside of thesecond center section to form connection ports for connection with thesecond pair of hydraulic lines. At least one of the first and secondcenter sections forms a charging passage having a first end opening tothe outside of the at least one of the first and second center sectionsto form an inlet port for charging, serving as an inlet for thehydraulic fluid to be replenished, and a second end communicating withthe first pair of hydraulic passages and the second pair of hydraulicpassages via check valves. The charging passage is connected to apressure relief line communicating with the inside of the housing via arelief valve, and the inlet port for charging is connected to thereservoir tank via a hydraulic fluid replenishing passage.

[0050] The pump unit of the fourth aspect of the present invention alsopreferably has the following arrangement. Specifically, the pump unitincludes a cooling fan provided near the single unit. The cooling fan isadapted to be driven in synchronism with the first and second hydraulicpumps. The reservoir tank is connected to the single unit in such amanner as to form a clearance therebetween, into which a cooling airstream is drawn from the cooling fan. The hydraulic fluid communicationpassage and the hydraulic fluid replenishing passage are disposed insuch a manner to traverse the clearance. The thus arranged pump unit canlimit the temperature increase of the hydraulic fluid stored in thereservoir tank and the housing, and also effectively limit thetemperature increase of the hydraulic fluid flowing through thehydraulic fluid replenishing passage and the hydraulic fluidcommunication passage, thereby improving the transmission efficiencybetween the hydraulic pumps and the actuators.

[0051] According to the fifth aspect of the present invention, there isprovided a pump unit for operation in association with first and secondactuators. The pump unit includes: a first hydraulic pump and a secondhydraulic pump respectively connected to the first and second actuatorsvia a first pair of hydraulic lines and a second pair of hydrauliclines; a center section supporting the first hydraulic pump and thesecond hydraulic pump; and a housing accommodating the first hydraulicpump and the second hydraulic pump. The housing is adapted to be used asa hydraulic fluid tank. A hydraulic fluid circulation mechanism is alsoprovided for taking the hydraulic fluid from the hydraulic tank, andagain returning the same to the hydraulic tank. The hydraulic fluidcirculation mechanism is designed to cool the hydraulic fluid whilecirculating the same.

[0052] The pump unit of the above arrangement can effectively limit theincrease in temperature of the hydraulic fluid stored within thehydraulic tank, thereby effectively preventing deterioration in workingefficiency of a hydraulic actuation device.

[0053] Preferably, the circulation mechanism of the pump unit of thefifth aspect of the present invention includes a circulation line, atleast a portion of which is formed by a conduit; the circulation linehaving a first end communicating with the inside of the hydraulic tankand a second end again communicating with the inside of the hydraulictank, and the conduit has at least a portion provided thereon withcooling fins.

[0054] The pump unit of the fifth aspect of the present inventionpreferably has the following arrangement. Specifically, the centersection is a unitary member on which the first and second hydraulicpumps are supported in parallel relationship with one another. Thehousing forms an opening in a side thereof, through which the first andsecond hydraulic pumps can pass. The center section and the housing areintegrally connected together to form a single unit, so that the openingof the housing can be sealed in a liquid tight manner by the centersection with the first and second hydraulic pumps supported thereon. Thecenter section forms a first pair of hydraulic passages respectivelyhaving first ends communicating with the first hydraulic pump and secondends opening to the outside of the center section to form connectionports for connection with the first pair of hydraulic lines, a secondpair of hydraulic passages respectively having first ends communicatingwith the second hydraulic pump and second ends opening to the outside ofthe center section to form connection ports for connection with thesecond pair of hydraulic lines, and a charging passage having a firstend communicating with the hydraulic fluid tank to form an inlet portfor charging, serving as an inlet for the hydraulic fluid to bereplenished and a second end communicating with the first pair ofhydraulic passages and the second pair of hydraulic passages via checkvalves. The pump unit further comprises: a charge pump for sucking thehydraulic fluid stored within the hydraulic fluid tank and thendischarging the same into the inlet port for charging; a pressure reliefline having a first end connected to the charging passage via a reliefvalve and a second end forming a drain port through which the hydraulicfluid from the relief valve is drained; and a pipe connecting the secondend of the pressure relief line with the hydraulic fluid tank; in whichthe pipe constitutes a conduit, and the charge pump constitutes a partof the hydraulic fluid circulation mechanism.

[0055] The pump unit of the fifth aspect of the present invention alsopreferably has the following arrangement. Specifically, the centersection includes a first center section and a second center sectionrespectively supporting the first and second hydraulic pumps. Thehousing has first and second sidewalls facing one another andrespectively forming a first opening and a second opening through whichthe first hydraulic pump and the second hydraulic pump can respectivelypass. The first and second center sections are integrally connected tothe housing to form a single unit, so that the first and second openingsof the housing are sealed in a liquid tight manner by the first andsecond center sections respectively supporting the first and secondhydraulic pumps thereon. The first center section forms a first pair ofhydraulic passages respectively having first ends communicating with thefirst hydraulic pump and second ends opening to the outside of the firstcenter section to form connection ports for connection with the firstpair of hydraulic lines. The second section forms a second pair ofhydraulic passages respectively having first ends communicating with thesecond hydraulic pump and second ends opening to the outside of thesecond center section to form connection ports for connection with thesecond pair of hydraulic lines. At least one of the first and secondcenter sections forms a charging passage having a first endcommunicating with the hydraulic fluid tank to form an inlet for thehydraulic fluid to be replenished and a second end communicating withthe first pair of hydraulic passages and the second pair of hydraulicpassages via check valves. The pump unit further comprises: a chargepump for sucking the hydraulic fluid stored within the hydraulic fluidtank and then discharging the same into the inlet port for charging; apressure relief line having a first end connected to the chargingpassage via a relief valve and a second end forming a drain port throughwhich the hydraulic fluid from the relief valve is drained; and a pipeconnecting the second end of the pressure relief line with the hydraulicfluid tank; in which the pipe constitutes a conduit, and the charge pumpconstitutes a part of the hydraulic fluid circulation mechanism.

[0056] The pump unit of the fifth aspect of the present inventionpreferably includes a reservoir tank, in which the reservoir tank is infree fluid communication with the housing via a hydraulic fluidcommunication passage, and forms a hydraulic fluid tank in cooperationwith the housing, in which the inlet port for charging communicates withthe reservoir tank via a hydraulic fluid replenishing passage.

[0057] The pump unit of the fifth aspect of the present inventionpreferably includes cooling fins provided on the hydraulic fluidreplenishing passage and the hydraulic fluid communication passage.

[0058] The pump unit of the fifth aspect of the present inventionpreferably has the following arrangement. Specifically, a cooling fanadapted to be driven in synchronism with the first and second hydraulicpumps is provided near the single unit. The reservoir tank is connectedto the single unit in such a manner as to form a clearance therebetween,into which a cooling air stream from the cooling fan is drawn. Thehydraulic fluid communication passage and the hydraulic fluidreplenishing passage are disposed to transverse the clearance.

[0059] A cooling air duct is preferably provided in the pump unit of thefifth aspect of the present invention, so that a cooling air stream fromthe cooling fan is drawn into the clearance along the cooling air duct.

BRIEF DESCRIPTION OF THE DRAWINGS

[0060] The above, and other objects, features and advantages of thepresent invention will become apparent from the detailed descriptionthereof in conjunction with the accompanying drawings wherein.

[0061]FIG. 1 is an expansion plan view of a vehicle to which a pump unitaccording to first to fifth aspects of the present invention areapplied.

[0062]FIG. 2 is a hydraulic circuit diagram of the vehicle to which oneembodiment of a pump unit according to the first aspect of the presentinvention is applied.

[0063]FIG. 3 is a cross sectional plan view of the pump unit accordingto the embodiment of FIG. 2.

[0064]FIG. 4 is a perspective view with a partially exploded portion ofthe pump unit of FIGS. 2 and 3.

[0065]FIG. 5 is a cross section taken along lines V-V in FIG. 3.

[0066]FIG. 6 is a cross section taken along lines VI-VI in FIG. 3.

[0067]FIG. 7 is a cross section taken along lines VII-VII in FIG. 3.

[0068]FIG. 8 is a cross section taken along lines VIII-VIII in FIG. 3.

[0069]FIG. 9 is a cross section taken along lines IX-IX in FIG. 3.

[0070]FIG. 10 is a cross section taken along lines X-X in FIG. 3.

[0071]FIG. 11 is a cross section taken along lines XI-XI in FIG. 6.

[0072]FIG. 12 is a plan view of the pump unit of FIGS. 2 and 3.

[0073]FIG. 13 is a longitudinal cross section of a first center sectionof another embodiment of the pump unit according to the first aspect ofthe present invention.

[0074]FIG. 14 is a cross sectional plan view illustrating a modifiedexample of a pump case in the pump unit of FIGS. 2 and 3.

[0075]FIG. 15 is a hydraulic circuit diagram of the vehicle to which oneembodiment of a pump unit according to the second aspect of the presentinvention is applied.

[0076]FIG. 16 is a longitudinal cross-sectional front view of the pumpunit illustrated in FIG. 15.

[0077]FIG. 17 is a cross section taken along lines XVII-XVII in FIG. 16.

[0078]FIG. 18 is a cross section taken along lines XVIII-XVIII in FIG.16.

[0079]FIG. 19 is a cross section taken along lines XIX-XIX in FIG. 16.

[0080]FIG. 20 is a longitudinal cross-sectional side view of the pumpunit according to another embodiment of the second aspect of the presentinvention.

[0081]FIG. 21 is a longitudinal cross-sectional front view of the pumpunit illustrated in FIG. 20.

[0082]FIG. 22 is a cross section taken along lines XXII-XXII in FIG. 20.

[0083]FIG. 23 is a cross section taken along lines XXII-XXII in FIG. 20.

[0084]FIG. 24 is a hydraulic circuit diagram of the vehicle to which oneembodiment of a pump unit according to the third aspect of the presentinvention is applied.

[0085]FIG. 25 is a longitudinal cross-sectional front view of the pumpunit illustrated in FIG. 24.

[0086]FIG. 26 is a cross section taken along lines XXVI-XXVI in FIG. 25.

[0087]FIG. 27 is a cross section taken along lines XXVII-XXVII in FIG.25.

[0088]FIG. 28 is a cross section taken along lines XXVIII-XXVIII in FIG.25.

[0089]FIG. 29 is a longitudinal cross-sectional side view of the pumpunit according to another embodiment of the third aspect of the presentinvention.

[0090]FIG. 30 is a longitudinal cross-sectional front view of the pumpunit illustrated in FIG. 29.

[0091]FIG. 31 is a cross section taken along lines XXXI-XXXI in FIG. 29.

[0092]FIG. 32 is a cross section taken along lines XXXII-XXXII in FIG.29.

[0093]FIG. 33 is a hydraulic circuit diagram of the vehicle to which oneembodiment of a pump unit according to the fourth aspect of the presentinvention is applied.

[0094]FIG. 34 is a longitudinal cross-sectional side view of the pumpunit illustrated in FIG. 33.

[0095]FIG. 35 is a cross section taken along lines XXXV-XXXV in FIG. 34.

[0096]FIG. 36 is a cross section taken along lines XXXVI-XXXVI in FIG.34.

[0097]FIG. 37 is an enlarged view of a portion XXXVII in FIG. 34.

[0098]FIG. 38 is a cross section taken along lines XXXVIII-XXXVIII inFIG. 37.

[0099]FIG. 39 is a cross section taken along lines XXXIX-XXXIX in FIG.37.

[0100]FIG. 40 is a cross section taken along lines XXXX-XXXX in FIG. 39.

[0101]FIG. 41 is a longitudinal cross-sectional side view of the pumpunit according to another embodiment of the fourth aspect of the presentinvention.

[0102]FIG. 42 is a cross section taken along lines XXXXII-XXXXII in FIG.41.

[0103]FIG. 43 is a hydraulic circuit diagram of the vehicle to which oneembodiment of a pump unit according to the fifth aspect of the presentinvention is applied.

[0104]FIG. 44 is a longitudinal cross-sectional side view of the pumpunit illustrated in FIG. 43.

[0105]FIG. 45 is a cross section taken along lines XXXXV-XXXXV in FIG.44.

[0106]FIG. 46 is a cross section taken along lines XXXXVI-XXXXVI in FIG.44.

[0107]FIG. 47 is a cross section taken along lines XXXXVII-XXXXVII inFIG. 44.

[0108]FIG. 48 is a cross section taken along lines XXXXVIII-XXXXVIII inFIG. 44.

[0109]FIG. 49 is a cross section taken along lines XXXXIX-XXXXIX in FIG.47.

[0110]FIG. 50 is a perspective view of a mounting member.

[0111]FIG. 51 is a longitudinal cross-sectional side view of the pumpunit according to another embodiment of the fifth aspect of the presentinvention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT First Embodiment

[0112] The first embodiment of the pump unit according to the firstaspect of the present invention will be hereinafter described withreference to the accompanying drawings.

[0113] A pump unit 100 according to the first aspect of the presentinvention is designed to be operated in association with an actuatorthat is connected thereto via first and second pairs of hydraulic lines184 a and 184 b and driven through an effect of pressurized hydraulicfluid in the pair of hydraulic lines. This embodiment will be describedby taking for example the case that hydraulic motors 182 a and 182 beach are used as the actuator.

[0114]FIGS. 1 and 2 are respectively an expansion plan view of a vehicleto which the pump unit 100 of this embodiment is applied, and ahydraulic circuit of the vehicle. FIG. 3 is a cross sectional plan viewof the pump unit and its periphery. FIG. 4 is a perspective view with apartially exploded portion of the pump unit. FIGS. 5 to 10 arerespectively cross sections taken along lines V-V, VI-VI, VII-VII,VIII-VIII, IX-IX, and X-X in FIG. 3. FIG. 11 is a cross section takenalong lines XI-XI in FIG. 6. The reference codes 185, 197 a and 197 b,and 199 in FIG. 1 respectively represent a reservoir tank, casterwheels, and a driver seat.

[0115] As illustrated in FIGS. 2 and 3, the pump unit 100 includes afirst hydraulic pump 110 a, a second hydraulic pump 100 b, a pump case120 that accommodates the first and second hydraulic pumps 100 a and 100b and has an opening 121 c through which the hydraulic pumps areinserted into the pump case 120, and a center section 130 connected tothe pump case so as to close the opening of the pump case.

[0116] In this embodiment, the pump unit 100 is explained as having apair of hydraulic pumps. However, the first aspect of the presentinvention is not necessarily limited to this arrangement. Rather, it isapplicable to the case where a single hydraulic pump is used, or threeor more than three hydraulic pumps are used.

[0117] As illustrated in FIG. 2, the first and second hydraulic pumps110 a and 110 b are of a variable displacement type, which has avariable input/output flow rates through the operation of a swash plate.The hydraulic pumps 110 a and 110 b are respectively connected to thefirst and second hydraulic motors 182 a and 182 b serving as theactuator, via the first pair of hydraulic lines 184 a and the secondpair of hydraulic lines 184 b.

[0118] Accordingly, varying the input/output flow rates of each of thehydraulic pumps 110 a and 110 b through the swash plate causes thepressure difference between the first pair of hydraulic lines 184 a, andthe second pair of hydraulic lines 184 b. According to the operationalangle of the swash plate, motor shafts of the first hydraulic motor 182a and/or the second hydraulic motor 182 b are rotated, and drive wheels183 a and 183 b that are operatively connected to the motor shafts aredriven. The reference codes 180 and 181 in FIG. 2 respectively representa power source and a cooling fan.

[0119] As described above, the first and second hydraulic pumps 110 aand 110 b according to this embodiment are of the variable displacementtype, and the first and second hydraulic motors 182 a and 182 b inassociation with the hydraulic pumps 110 a and 11 b are of a fixeddisplacement type. However, the first aspect of the present invention isnot necessarily limited to this arrangement. That is, it is possible toemploy the hydraulic pumps of the fixed displacement type, and thehydraulic motors of the variable displacement type driven by thehydraulic pumps or the hydraulic pumps and the hydraulic motors, both ofwhich are of the variable displacement type.

[0120] In this embodiment, the first and second hydraulic pumps 110 aand 110 b are of an axial piston type. Alternatively, the pump unit mayemploy the hydraulic pumps of a radial piston type.

[0121] As illustrated in FIGS. 3 and 5, the first and second hydraulicpumps 110 a and 110 b respectively include a first hydraulic pump shaft111 a and a second hydraulic pump shaft 111 b, both of which aredisposed parallel to one another, a first piston unit 112 a and a secondpiston unit 112 b that are reciprocatingly movable according to therotation of the pump shafts, a first cylinder block 113 a and a secondcylinder block 113 b that reciprocably support the piston units, a firstangularly adjustable swash plate 114 a and a second angularly adjustableswash plate 114 b that regulate the stroke lengths of the piston unitsby varying their tilting angles to vary their input/output flow rates,and a first control shaft 115 a and a second control shaft 115 b thatcontrol the tilting angles of these swash plates.

[0122] As illustrated in FIG. 5, the first control shaft 115 a has aninner end extending into the pump case 120 and connected to the firstangularly adjustable swash plate 114 a via an arm 116 a, and an outerend extending vertically above the pump case 120. The second controlshaft 115 b has a similar arrangement (not shown).

[0123] In this embodiment, the pump unit 100 is of a horizontal typethat has the horizontally extending first and second hydraulic pumpshafts 111 a and 111 b. However, the first aspect of the presentinvention is not necessarily limited to this arrangement. Rather, it isa matter of course to employ the pump unit of a vertical type that hasthe vertically extending first and second hydraulic pump shafts 111 aand 111 b.

[0124] The pump unit 100 further includes a neutral position returnmechanism 150 that returns the swash plates 114 a and 114 b of the firstand second hydraulic pumps 110 a and 110 b to their respective neutralpositions. A plan view of a portion of the pump unit 100 is illustratedin FIG. 12.

[0125] The neutral position return mechanism 150 includes a firstneutral position return mechanism 150 a for the first hydraulic pump 110a, and a second neutral position return mechanism 150 b for the secondhydraulic pump 110 b disposed on a common base plate 150 c mounted onthe upper surface of the pump case 120. The following description, whichwill be made for the first neutral position return mechanism 150 a, willalso be applicable for the second neutral position return mechanism 150b.

[0126] As illustrated in FIG. 12, the first neutral position returnmechanism 150 a includes a connecting arm 151 with a first end pivotablyconnected to a connecting member 192 a connected to a control lever 198a (see FIG. 1) and a second end non-rotatably connected to the controlshaft 115 a, a swing arm 152 a with a proximal end non-rotatablyconnected to the control shaft 115 a and a distal end as a free end, andan eccentric pin 153 a detachably fixed to the upper surface of the pumpcase 120.

[0127] The swing arm 152 a has a distal edge defining a deepest part 193a near the shaft center of the control shaft 115 a, and cam surfaceslaterally extending from the deepest part in such a manner as togradually increase the distance from the shaft center of the controlshaft 115 a as they extend away from the deepest part.

[0128] The eccentric pin 153 a has a first shaft portion 195 a mountedon the upper surface of the base plate 150 c, and a second shaft portion196 a extending upwardly from the first shaft portion 195 a. The secondshaft portion has the shaft center eccentric to that of the first shaftportion, so that the second shaft portion 196 a has the shaft centerrotated around the shaft center of the first shaft portion 195 a throughthe rotation of the first shaft portion 195 a around the shaft centerthereof.

[0129] The first neutral position return mechanism 150 a furtherincludes a pressing arm 154 a with a proximal end rotatably supported onthe second shaft portion 196 a of the eccentric pin 150, and a distalend as a free end. The pressing arm 154 a is provided with a roller 155a engageable with the distal edge of the swing arm 152 a. The pressingarm 154 a and the swing arm 152 a are disposed in such a manner as tohave the roller 155 a engageable with the deepest part 193 a of theswing arm 152 a, when the swash plate of the hydraulic pump 110 a liesat the neutral position.

[0130] Specifically, the pressing arm 154 a is positioned such that theroller 155 a is engaged with the deepest part 193 a of the swing arm 152a, while holding the swash plate of the hydraulic pump 110 a at aposition to be considered as the neutral position. At this time, theremay occur the case where the hydraulic pump 110 a is not brought into aneutral state due to assembling errors or the like. Specifically, theremay occur the case the swing arm 152 a must be rotated by apredetermined angle in either direction around the shaft center of thecontrol shaft 115 a from a predetermined design position illustrated inFIG. 12, in order to have the swash plate lying at the neutral position.

[0131] Even if such a positional error occurs, the first neutralposition return mechanism 150 can adjust the relative displacementbetween the control shaft 115 a and the connecting member 192 a easily.Specifically, in the first neutral position return mechanism 150, theeccentric pin 153 a has the second shaft portion 196 a as the supportingpoint of the pressing arm 154 a, which shaft portion is eccentric to thefirst shaft portion 195 a, thereby allowing the second shaft portion 196a to have the shaft center easily adjustable through the rotation of thefirst shaft portion 195 a around the shaft center thereof, and hence theroller 155 a to have an easily adjustable distance relative to the swingarm 152 a. Thus, the roller 155 a can easily be brought into engagementwith the deepest part 193 a of the swing arm 152 a, even when thehydraulic pump 110 a cannot come into the neutral state without therotation of the swing arm 152 a by a predetermined angle around theshaft center of the control shaft 115 from the predetermined designposition.

[0132] The first neutral position return mechanism 150 a furtherincludes a spring member 156 a to urge the roller 155 a towards thedistal edge of the swing arm 152 a.

[0133] The first neutral position return mechanism 150 a having theabove arrangement performs in the following manner. When the driveroperates the mechanism 150 a through the shifting operation of a controllever 198 a provided near the driver seat, the connecting member 192 ais slidingly moved along either direction (F or R with N therebetween)indicated by the arrows of FIG. 12 according to the shifting operationof the control lever 198 a, thereby pivotally moving the connecting arm151 a, and hence rotating the control shaft 115 a. The swash plate canbe thus tilted according to the shifting operation of the control lever.

[0134] On the other hand, when the driver releases the control lever 198a from its operational state where the swash plate is held in a tiltedposition, the first neutral position return mechanism 150 a canautomatically return the swash plate of the hydraulic pump to theneutral position. Specifically, since the swash plate tilted in eitherdirection from the neutral position causes the control shaft 115 a to berotated around the shaft center thereof in either direction from theneutral position, the swing arm 152 a is pivotally moved in eitherdirection around the shaft center of the control shaft 115 a accordingto the rotation angle of the control shaft 115 a. Accordingly, theroller 155 a is engaged with one of the cam surfaces defined by thedistal edge of the swing arm 152 a. As described above, since the roller155 a is constantly urged towards the distal edge of the swing arm 152 aby the spring member 156 a, the swing arm 152 a automatically returns tothe neutral position, at which the roller 155 a is engaged with thedeepest part 193 a, through a camming effect between the roller 155 aand the cum surface 194 a by releasing the control lever from theoperational state where the roller 155 a is engaged with the cum surface194 a.

[0135] Thus, the first neutral position return mechanism 150 a performsso that the swash plate of the hydraulic pump 110 a automaticallyreturns to the neutral position in response to releasing the controllever 198 a from the operational state.

[0136] The second neutral position return mechanism 150 b has a similararrangement. Accordingly, the right and left control levers 198 a and198 b are controlled independently of each other, so that the first andsecond hydraulic pumps can have the swash plates tilted independently ofeach other. Both control levers 198 a and 198 b are released from theoperational states to stop the vehicle without any delay.

[0137] As best illustrated in FIG. 3, the pump unit 100 further includesa power transmission mechanism 158 that is accommodated within thehousing 120 to operatively connect the first hydraulic pump shaft 111 ato the second hydraulic pump shaft 111 b.

[0138] The power transmission mechanism 158 provided in the pump unit100 can simultaneously drive both hydraulic pump shafts 111 a and 111 bonly by connecting the power source 180 to either one of the firsthydraulic pump shaft 111 a and the second hydraulic pump shaft 111 b,resulting in a simple transmission arrangement between the power source180 and the pump unit 100.

[0139] In this embodiment, the power transmission mechanism 158 is inthe form of a gear transmission device that includes a first gear 158 anon-rotatably supported on the first hydraulic pump shaft 111 a, and asecond gear 158 b non-rotatably supported on the second hydraulic pumpshaft 111 b in meshed engagement with the first gear 158 a. Instead ofthe gear transmission device, any conventional power transmissionmechanisms such as chain and belt may be used.

[0140] The pump case 120 is sealed in a liquid tight manner by thecenter section 130, thereby reserving the hydraulic fluid within thepump case 120. Specifically, the pump case 120 also serves as a part ofa reservoir tank. The reference code 145 in FIGS. 4 and 7 represents ahole used along with a separately provided hydraulic fluid tank tocommunicate the inside of the pump case with the hydraulic fluid tank.

[0141] As illustrated in FIG. 3, the pump case 120 includes a first pumpcase 121 for accommodating the first and second hydraulic pumps 110 aand 110 b, and a second pump case 122 for accommodating the powertransmission mechanism 158.

[0142] As best illustrated in FIGS. 3 and 5, the first pump case 121 hasa box shape with a first side wall 121 a disposed in either side alongthe longitudinal direction of the hydraulic pump shafts 111 a and 111 b,or in this embodiment in the front side of the vehicle, which will behereinafter referred to simply as the front side, and a peripheral wall121 b extending from a peripheral edge of the first sidewall 121 a tothe opposite side of the pump unit 100 along the longitudinal directionof the hydraulic pump shafts 111 a and 111 b (i.e., the rear side of thevehicle in this embodiment, which will be referred to simply as the rearside). The first sidewall 121 a forms bearing holes through which thefirst and second hydraulic pump shafts 111 a and 111 b respectivelyextend. The rear side has an end surface defining the opening 121 c forreceiving the first and second hydraulic pumps 110 a and 110 b. Theopening of the pump case 121 is sealed by the center section 130 in aliquid tight manner.

[0143] The second pump case 122 has a box shape with a front sidewall122 a and a peripheral wall 122 b extending from a peripheral edge ofthe front sidewall 122 a to the rear side to form a box shape. The frontsidewall 122 a forms a bearing hole through which the front end portionof the first hydraulic pump shaft 111 b extend, and a bearing portionfor supporting the front end portion of the second hydraulic pump shaft111 b. The rear side of the second pump case 122 has an end surfaceforming an opening 122 c for receiving the power transmission mechanism150.

[0144] The second pump case 122 is connected to the first pump case 121so that the opening 122 c can be sealed by the first sidewall 121 a ofthe first pump case 121 in a liquid tight manner, and forms anaccommodation space of the power transmission mechanism 158 incooperation with the first sidewall 121 a of the first pump case 121.

[0145] In the thus arranged pump case 120, the first sidewall 121 a ofthe first pump case 121 serves as a partition wall dividing the pumpcase accommodation space into a hydraulic pump accommodation chamber anda power transmission mechanism accommodation chamber. The partition wallthus defining the hydraulic pump accommodation chamber and the powertransmission mechanism accommodation chamber can effectively prevent anyforeign matters such as iron powder generated in the power transmissionmechanism 158 from intruding into the hydraulic pump accommodationchamber, and hence damaging piston units 112 a, 112 b, cylinder blocks113 a, 113 b and/or other parts. A seal ring, oil seal or the like mayalso be provided around the peripheral surface of the first and secondhydraulic pump shafts 111 a and 111 b extending through the partitionwall 121 a to more securely prevent the intrusion of the foreignmatters.

[0146] Portions of the pump case 120, through which the respectiveshafts 111 a, 115 a and 115 b extend, are sealed by any suitable sealingmeans in a liquid tight manner, thereby allowing the pump case 120 toserve as a hydraulic fluid tank.

[0147] As illustrated in FIG. 6, the first sidewall 121 a serving as thepartition wall more preferably forms a hydraulic fluid communicationhole 123 for communication between the hydraulic pump accommodationchamber and the power transmission mechanism accommodation chamber witha filter 124 provided in the hole for preventing the intrusion of theforeign matters into the hydraulic pump accommodation chamber. The thusformed hydraulic fluid communication hole 123 can omit the necessity ofseparately feeding lubricant to the power transmission mechanism 158,with the result that the power transmission mechanism 158 can belubricated with the hydraulic fluid stored within the pump case 120.This permits low manufacturing cost and ease of maintenance.

[0148] Hydraulic fluid communication holes 123 are preferably andrespectively be provided with the meshed point of the first gear 158 aand the second gear 158 b therebetween, and more particularly at thedownstream and upstream sides with respect to the rotational directionof the first gear 158 a and the second gear 158 b. The thus arrangedhydraulic fluid communication holes 123 achieve an efficient circulationof the hydraulic fluid between the hydraulic pump accommodation chamberand the power transmission mechanism accommodation chamber.

[0149] In this embodiment, the first and second angularly adjustableswash plates 114 a and 114 b are of a cradle type, as illustrated inFIG. 3. Therefore, when the partition wall 121 a forms, on its sidefacing the hydraulic pumps 110 a, 110 b, spherical concave surfaces 126a and 126 b respectively adapted to spherical convex surfaces 116 a and116 b formed in the rear portions of the swash plates 114 a and 114 b,which rear portions being opposite to the surfaces facing the pistonunits 112 a and 112 b, the spherical concave surfaces 126 a and 126 bcan slidingly guide the spherical convex surfaces 116 a and 116 b of theswash plates 114 a and 114 b thereon. The swash plates thus can besecurely positioned on the spherical concave surfaces 126 a and 126 b.

[0150] In this embodiment, the first sidewall 121 a of the first pumpcase 121 serves as the partition wall. Alternatively, a partitioningmeans may take various forms, as long as it can produce the same effect.For example, a separately prepared partition wall 121 a′ may be mountedin a first pump case 121′ having a simple box shape, as illustrated inFIG. 14. This arrangement is advantageous in that the spherical concavesurfaces 116 a and 116 b can easily be formed.

[0151] Now, the description will be made for the center section 130. Asbest illustrated in FIGS. 3 and 5, the center section 130 includes afirst center section 131 connected to the pump case 120 through directcontact to the rear side of the pump case 120 for sealing the opening121, and a second center section 132 connected to the first centersection 131 in such a manner as to surround a charge pump 160operatively driven by the hydraulic pump shaft 111 a of the firsthydraulic pump 110 a.

[0152] The center section 130, as illustrated in FIGS. 4, 5 and 7, formsa first pair of inlet/outlet passages 133 a having first endsrespectively communicating with inlet and outlet ports of the firsthydraulic pump 110 a and second ends opening to the outside of thecenter section 130 through a pump-case abutting surface 131 a of thefirst center section 131, and a second pair of inlet/outlet passages 133b having first ends respectively communicating with inlet and outletports of the second hydraulic pump 110 b and second ends opening to theoutside of the center section 130 through the pump-case abutting surface131 a of the first center section 131. Both pairs of the passages 133 aand 133 b are disposed parallel with each other.

[0153] The center section 130 forms a first charging passage 134 havinga first end communicating with an outlet port of the charge pump 160,and a second end formed in the pump-case abutting surface 131 a of thefirst center section 131.

[0154] The first pair of inlet/outlet passages 133 a constitutes a partof the first pair of hydraulic lines 184 a communicating between thefirst hydraulic pump 110 a and the first hydraulic motor 183 a. On theother hand, the second pair of inlet/outlet passages 133 b constitutes apart of the second pair of hydraulic lines 184 b communicating betweenthe second hydraulic pump 110 b and the second hydraulic motor 183 b(see FIG. 2).

[0155] As illustrated in FIG. 2, the center section 130 forms a pressurerelief line 135 communicating with the first charging passage 134. Thepressure relief line 135 is provided therein with a relief valve 161 foradjusting the hydraulic pressure of the first charging passage 134. Inthis embodiment, the relief valve 161 is disposed in the second centersection 132, as illustrated in FIGS. 10 and 11.

[0156] At least one of the pump case 120 and the center section 130forms a communication passage communicating between the first pair ofinlet/outlet passages 133 a and the second pair of inlet/outlet passages133 b, and the first charging passage 134 via their second ends.

[0157] In this embodiment, a first center section abutting surface 121 eof the first housing, as best illustrated in FIGS. 4 and 8, forms asingle fluid groove 136 extending over the second ends of the first pairof inlet/outlet passages 133 a and the second pair of inlet/outletpassages 133 b. The first charging passage 134 has the second endcommunicating with the fluid passage groove 136.

[0158] Charging check valves 162 a, 162 b, 162 c and 162 d are providedin such a manner as to be installable through the abutting surface 131 aof the first center section 131 or the abutting surface 121 e of thefirst housing 121, as best illustrated in FIG. 4. These valves aredesigned to allow the flow of the hydraulic fluid from the firstcharging passage 134 to the first pair of inlet/outlet passages 133 aand the second pair of inlet/outlet passages 133 b, while preventing thereverse flow.

[0159] The installation of those charging check valves through the firstcenter section abutting surface 121 e of the first housing 121 or thefirst housing abutting surface of the first center section 131 producesthe following effects.

[0160] Specifically, since a die pattern of the fluid passage groove 136can be formed in a die for the first housing 121 or the first centersection 121, it is not necessary to additionally bore holes forreceiving the charging check valves 162 a to 162 d. This omits thenecessity of a conventionally required machining process, resulting in alower manufacturing cost.

[0161] Since the charging check valves 162 can be secured in positiononly by connecting the first housing 121 to the first center section131, a covering member or other check valve fixing means can be omitted.This arrangement can reduce the number of parts, resulting in a lowermanufacturing cost and an improved assembling efficiency.

[0162] As illustrated in FIGS. 4, 8 and 11, the center section abuttingsurface 121 e of the pump case 120 forms an escape groove 137surrounding the fluid passage groove 136 and having at least oneterminal portion communicating with the inside of the pump case for thedischarge of the leaked hydraulic fluid.

[0163] With the escape groove 137, the hydraulic fluid, which flows fromthe first charging passage 134 to the first pair of inlet/outletpassages 133 a and the second pair of inlet/outlet passages 133 b viathe fluid passage groove 136, is prevented from leaking to the outsidethrough the abutting portion between the first pump case 121 and thefirst center section 131. Specifically, the hydraulic fluid leaked outof the fluid passage groove 136 is held in the escape groove 137, andthen returned to the inside of the pump case 120. Whereby, the leakageof the hydraulic fluid outwards of the case from the abutting portionbetween the first pump case 121 and the first center section 131 caneffectively be prevented.

[0164] Leak lines 163 a and 163 b each having a throttle valve arepreferably formed between the first charging passage 134 and at leastone of the first pair of inlet/outlet passages 133 a, and between thefirst charging passage 134 and at least one of the second pair ofinlet/outlet passages 133 b (see FIG. 2).

[0165] The leak lines 163 a and 163 b are designed to assure theneutralization of the hydraulic pumps 110 a and 110 b. Specifically,even if the swash plates 114 a and 114 b of the hydraulic pumps 110 aand 110 b tilt from the neutral positions by a small angle, there occursthe pressure difference between the first pair of hydraulic lines 184 a,and/or between the second pair of hydraulic lines 184 b. This pressuredifference causes the rotation of the hydraulic motors 182 a and 182 b.That is, even a slight amount of the displacement between the actualneutral positions and the predetermined design positions of the swashplates 114 a and 114 b due to assembling error or the like causes anunintentional rotation of the hydraulic motors 182 a and 182 b. On thecontrary, the leak lines 163 a and 163 b, as described above, allow thepressurized hydraulic fluid to leak therethrough from the first pair ofhydraulic lines 184 a or the second pair of hydraulic lines 184 b. Thus,the swash plates can have the neutral positions of a broadened effectivearea by effectively limiting the pressure difference between the pair offirst hydraulic lines 184 a, and/or between the second pair of hydrauliclines 184 b, thereby effectively avoiding the unintentional rotation ofthe hydraulic motors 182 a and 182 b, even for the swash plates 114 aand 114 b having the actual neutral position displaced from the designneutral position due to the assembling errors or the like.

[0166] In view of transmission efficiency between the hydraulic pumps110 a, 110 b and the hydraulic motors 182 a, 182 b, the leakage of thepressurized hydraulic fluid from the first and second pairs of hydrauliclines 184 a, 184 b through the leak lines 163 a, 163 b is notpreferable. Therefore, the leak lines 163 a, 163 b are preferablyprovided in portions from the first charging passage 133 to one of thefirst pair of inlet/outlet passages 133 a, and to one of the second pairof inlet/outlet passages 133 b, and more preferably to one of the firstpair of inlet/outlet passages 133 a which has a higher pressure duringrearward movement of the vehicle. This is because the forward movementof the vehicle frequently occurs as compared with the rearward movement.

[0167] The first center section 131, as illustrated in FIGS. 2 and 7,forms a first bypass passage 138 a for communication between the firstpair of inlet/outlet passages 133 a, and a second bypass passage 138 bfor communication between the second pair of inlet/outlet passages 133b. Although the following description will be made for the first bypasspassage 138 a, it is also applicable for the second bypass passage 138b.

[0168] In this embodiment, the first pair of inlet/outlet passages 133 aare formed parallel to one another, and the first bypass passage 138 ais formed orthogonal to the pair of first inlet/outlet passages, as bestillustrated in FIG. 7. This arrangement achieves the communicationbetween the first pair of inlet/outlet passages 133 a by forming only asingle hole.

[0169] The first bypass passage 138 a includes a first bypass valve 140a adapted to take a communication position and a cutoff positionrespectively for bringing the first pair of inlet/outlet passages 133 ainto and out of communication with one another. The first bypass valve140, as illustrated in FIG. 7, has a proximal end portion 141 aextending to the outside of the first center section 131 to be operatedfrom the outside of the first center section 131, allowing the firstbypass valve 140 a to take the communication position and the cutoffposition.

[0170] Specifically, the first bypass passage 138 a includes an inwardlythreaded portion 139 a having a proximal end opening to the outside ofthe first center section 131 and an inner threaded circumferentialsurface, a middle portion 139 b extending inwardly from the inner end ofthe inwardly threaded portion 139 a in such a manner as to straddle anadjacent one of the first pair of inlet/outlet passages 133 a, a distalend portion 139 c having a diameter smaller than the middle portion 139b with a stepped portion and communicating with a remote one of thefirst pair of inlet/outlet passages 133 a.

[0171] On the other hand, the first bypass valve 140 a includes aproximal end portion 141 a lying outside of the first center section131, an outwardly threaded portion 141 b distally extending from theproximal end portion 141 a and having an outer threaded circumferentialsurface for threaded engagement with the inner threaded circumferentialsurface 139 a, a seal portion 141 c distally extending from theoutwardly threaded portion 141 b and liquid-tightly engageable with themiddle portion 139 b at the proximal side with respect to the adjacentone of the first pair of inlet/outlet passages 133 a, and an abuttingportion 141 d distally extending from the seal portion 141 c and havinga shape adapted to the stepped portion for sealed contact between theabutting portion and the stepped portion. The first bypass valve 140 athus can take the cutoff position with the abutting portion 141 dabutting the stepped portion, and the communication position with theabutting portion 141 d located away from the stepped portion, throughthe axial sliding motion of the first bypass valve 140 a caused by therotation of the first bypass valve 140 a around the axis thereof via theproximal end portion 141 a.

[0172] A release means including the first bypass passage 138 a and thefirst bypass valve 140 a, as well as the second bypass passage 138 b andthe second bypass valve 140 b is designed to easily move the vehicle,when the vehicle must forcibly be moved or the vehicle wheels mustforcibly be rotated by man power or the like due to the disorder of thepower source 180, the hydraulic pumps 110 a, 110 b or the like.Specifically, when the vehicle wheels connected to the hydraulic motors182 a and 182 b are forcibly rotated with the first pair of hydrauliclines 184 a and/or the second pair of hydraulic lines 184 b lying in theclosing state, there occurs the pressure difference between the firstpair of hydraulic lines 184 a, and between the second pair of hydrauliclines 184 b. As a result, the vehicle is hardly moved, or the vehiclewheels are hardly rotated. On the contrary, the release means can easilyachieve the communications between the first pair of hydraulic lines 184a, and between the second pair of hydraulic lines 184 b without thenecessity of mechanically releasing all the check valves 162 a to 162 d.Whereby, the vehicle can easily be moved by man power or the like.

[0173] In this embodiment, the first pair of inlet/outlet passages 133 aand the second pair of inlet/outlet passages 133 b, as illustrated inFIG. 7, respectively have connecting ports formed in the same side ofthe first center section 31, resulting in an easy piping work betweenthese connecting ports and the hydraulic motors 182 a and 182 b.

[0174] As illustrated in FIGS. 2 and 5, the first center section 131 andthe second center section 132 forms a second charging passage 142 with afirst end communication with the inside of the pump case 120 and asecond end communicating with the first charging passage 134. The secondcharging passage 142 is designed to prevent the flow of the hydraulicfluid from the first charging passage 134 to the pump case 120, and feedan additional amount of the hydraulic fluid from the pump case 120 tothe first and second pairs of hydraulic lines 184 a and 184 b when theselines have a reduced amount of the hydraulic fluid.

[0175] In this embodiment, the second charging passage 142 includes acheck valve 143 for allowing the flow of the hydraulic fluid from thepump case 120 to the first charging passage 134 while preventing thereverse flow, thereby producing the above mentioned effect. Although thecharge pump 160 may be somewhat deteriorated in operation efficiently,it is possible to employ a throttle valve instead of the check valve143.

[0176] Providing the second charging passage 142 can effectively preventthe vehicle from being brought into a so-called free wheel phenomenon,which occurs when the vehicle stopping on a sloping road is accidentallymoved downwardly, causing the rotation of the wheels. That is, for thestopped vehicle, the hydraulic pumps 110 a and 110 b have the swashplates lying at the neutral position. In this state, when the vehicle isstopped on, for example, a slopped road, the vehicle is subjected to aforce possibly causing the rotation of the wheels through the vehicleweight, or the rotation of the motor shafts of the hydraulic motors 182a and 182 b. Since the hydraulic pumps 110 a and 110 b are set in theneutral state, the hydraulic pumps 182 a and 182 b subjected to suchforce causes one of the first pair of hydraulic lines 184 a and one ofthe second pair of hydraulic lines 184 b to have a highly pressurizedhydraulic fluid, and the remaining ones to have a low pressurizedhydraulic fluid. The hydraulic fluid exceeding a predetermined pressurelevel in the hydraulic lines causes the leakage through a gap incylinder blocks of the hydraulic pumps in communication with such highlypressurized hydraulic lines. Whereby, the amounts of the hydraulicfluids in the pairs of hydraulic lines are reduced, facilitating thefree rotation of the motor shafts easier.

[0177] On the contrary, the second charging passage 142 of thisembodiment sucks the hydraulic fluid within the pump case when the firstpair of hydraulic lines 184 a has reduced amount of the hydraulic fluid,resulting in a negative pressure in these lines. Specifically, thesecond charging passage 142 can prevent the pair of the hydraulic linesfrom having a reduced amount of the hydraulic fluid, and hence the motorshafts from being freely rotated.

[0178] As illustrated in FIGS. 3, 6, 9 and 10, the second center section132 is provided at its rear end with a filter 186. The second centersection 132 forms an inlet line 165 having a first end communicationwith the charge pump 160 a through its inlet opening and a second endcommunicating with the filter 186, and a filter line 166 having a firstend communicating with the filter 186 and a second end communicatingwith a hydraulic fluid tank (not shown), thereby allowing the hydraulicfluid fed from the tank and passing through the filter 186 and thefilter line 166 to be sucked into the charge pump 160 through the inletopening.

[0179] The pump unit 100 of this embodiment constitutes a single unit byunitedly connecting the first and second hydraulic pumps 110 a and 110b, the center section 130 and the housing 120 together. Therefore, bothfirst and second pumps 110 a and 110 b can be installed on the vehicleonly by mounting the single unit on the vehicle, resulting in animproved efficiency in assembling the vehicle.

Second Embodiment

[0180] The second embodiment of the first aspect of the presentinvention will be described with reference to FIG. 13. FIG. 13 is alongitudinal cross section of a first center section 131′ of a pump unit100 according to this embodiment, which figure corresponding to FIG. 7illustrating the aforementioned first embodiment.

[0181] In this embodiment, the first bypass line 138 a and the secondbypass line 138 b are replaced by a single common line 138. In thefollowing description, corresponding or identical parts to those of thefirst embodiment have been given the same reference characters or thosewith primes (′) to omit a detailed description thereof.

[0182] The common bypass line 138′ has a proximal end portion openingoutwardly and a distal end portion communicating with all the first andsecond pairs of inlet/outlet passages 133 a and 133 b.

[0183] The common bypass line 138′ includes a single bypass valve 140′to be operated from the outside of the first center section 131′ for thecommunication and the cutoff of the hydraulic fluid between the firstpair of inlet/outlet passages 133 a, and between the second pair ofinlet/outlet passages 133 b.

[0184] In addition to the desirable effects produced by the firstembodiment, the pump unit of the second embodiment can achieve aneffective boring operation and lower manufacturing cost through thereduction of the number of parts.

[0185] In each of the aforementioned embodiments, the description hasbeen made for the case that a pair of hydraulic pumps is included.However, it is not necessary to limit the number of the hydraulic pumpsto that of these embodiments. The present invention is applicable to thearrangement where a single hydraulic pump, or more than two hydraulicpumps are included.

Third Embodiment

[0186] One embodiment of the pump unit according to the second aspect ofthe present invention will be hereinafter described with reference toFIGS. 15 to 19. FIG. 15 is a hydraulic circuit diagram of the vehicle towhich a pump unit 200 of this embodiment is applied. FIG. 16 is alongitudinal cross-sectional front view of the pump unit and itsperiphery. FIGS. 17 to 19 are respectively cross sections taken alonglines XVII-XVII, XVIII-XVIII, and XIX-XIX.

[0187] As illustrated in FIGS. 15 to 17, the pump unit 200 is adapted tobe used in a vehicle having right and left drive wheels 283 a and 283 bto which first and second hydraulic motors 282 a and 282 b arerespectively connected, and includes a first hydraulic pump 210 a and asecond hydraulic pump 210 b respectively connected to the first andsecond hydraulic motors 282 a and 282 b via a first pair of hydrauliclines 284 a and a second pair of hydraulic lines 284 b, and a commonhousing 220 for accommodating these hydraulic pumps 210 a and 210 b.

[0188] The connection form between the right and left drive wheels 283 aand 283 b, and the first and second hydraulic motors 282 a and 282 bmeant in this embodiment includes the direct connection of the drivewheels respectively to those hydraulic motors, and also an operativeconnection of the drive wheels respectively to the hydraulic motors viaa suitable power transmission mechanism. In FIG. 15, the reference codes280, 281 and 285 respectively represent a power source, a cooling fanand a hydraulic fluid tank.

[0189] As illustrated in FIGS. 16, 17 and 19, the first hydraulic pump210 a and the second hydraulic pump 210 b are axial piston pumps of avariable displacement type, and respectively include a first pump shaft211 a and a second pump shaft 211 b that have vertical axes and aredisposed parallel to one another in the vehicle width direction withinthe housing 220, a first piston unit 212 a and a second piston unit 212b that are reciprocatingly movable according to the rotation of the pumpshafts, a first cylinder block 213 a and a second cylinder block 213 bthat reciprocably support the piston units, a first angularly adjustableswash plate 214 a and a second angularly adjustable swash plate 214 bthat regulate the stroke length of the piston units by varying theirtilting angles to vary the input/output flow rates of the piston units,and a first control shaft 215 a and a second control shaft 215 b thatcontrol the tilting angles of these swash plates.

[0190] The pump unit of this embodiment is of a vertical type with thefirst and second pump shafts 211 a and 211 b having the vertical axes.However, the second aspect of the present invention is not necessarilylimited to this arrangement. It is a matter of course that the pump unit200 can be of a horizontal type with the first and second pump shafts211 a and 211 b having the horizontal axes.

[0191] As best illustrated in FIG. 16, the first and second angularlyadjustable swash plates 214 a and 214 b of this embodiment are of acradle type.

[0192] As illustrated in FIGS. 16 and 19, the first control shaft 215 aand the second control shaft 215 b extend away from one another in thevehicle width direction to respectively have oppositely positioned outerends, and inner ends extending into the housing 220 to be respectivelyconnected to arms 216 a and 216 b and hence the first and second swashplates 214 a and 214 b. The pump unit 200 with the thus arranged firstand second control shafts 215 a and 215 b is advantageous when installedon the vehicle having push-pull control levers 198 a and 198 b asillustrated in FIG. 1, since the first and second control shafts 215 aand 215 b can have the rotating shaft centers disposed parallel to thelongitudinal axis of the control levers, thereby achieving thesimplification of a link mechanism between these control shafts and thecontrol levers.

[0193] The first control shaft 215 a and the second control shaft 215 bare more preferably located at substantially the same position withrespect to the vehicle longitudinal direction, as illustrated in FIG.16. The thus arranged first and second control shafts 215 a and 215 bcan be aligned with the control levers in the vehicle width direction,thereby achieving a more simplified structure of the link mechanism.

[0194] The pump unit 200 further includes a common center section 230that supports the first and second hydraulic pumps 210 a and 210 b, anda power transmission mechanism 240 that is accommodated within thehousing 220 to operatively connect the first and second hydraulic pumpshafts 211 a and 211 b together.

[0195] The pump unit 200 with the power transmission mechanism 240permits the simultaneous rotation of both pump shafts 211 a and 211 bonly by connecting the power source to either one of the first andsecond pump shafts 211 a and 211 b, or to the first pump shaft 211 a inthis embodiment, thereby achieving the simplified structure for thepower transmission from the power source to the pump unit 200. In thisembodiment, the power transmission mechanism 240 is in the form of agear transmission device that includes a first gear 240 a non-rotatablysupported on the lower side of the first pump shaft 211 a, and a secondgear 240 b non-rotatably supported on the lower side of the second pumpshaft 211 b in meshed engagement with the first gear 240 a. Instead ofthe gear transmission device, any conventional power transmissionmechanisms such as chain and belt may be used.

[0196] The housing 220, as illustrated in FIGS. 16 and 17, includes afirst housing 221 for accommodating the first and second hydraulic pumps210 a and 210 b, and a second housing 225 for accommodating the powertransmission mechanism 240.

[0197] The first housing 221 has a box shape with a first sidewall 222disposed in the upper or lower side of the pump shafts 211 a and 211 balong the longitudinal direction thereof, or in this embodiment in thelower side of the pump shafts 211 a and 211 b, which will be hereinafterreferred to simply as the lower side, and a peripheral wall 223extending from a peripheral edge of the first sidewall 222 to theopposite side of the pump shafts 211 a and 211 b along the longitudinaldirection thereof (i.e., the upper side of the pump shafts 211 a and 211b in this embodiment, which will be referred to simply as the upperside). The first sidewall 222 forms bearing holes through which thefirst and second pump shafts 211 a and 211 b respectively extend. Theupper side of the first housing 221 has an end surface forming anopening through which the first and second hydraulic pumps 210 a and 210b can be placed into the first housing 221. The opening of the firsthousing 221 is sealed by the center section 230 in a liquid tightmanner. That is, the center section 230 of this embodiment constitutes apart of the first housing 221. The first and second control shafts 215 aand 215 b extend away from one another in the vehicle width direction torespectively have outer ends protruding from the peripheral wall 223 ofthe first housing 221.

[0198] The second housing 225 is disposed in the lower side, and has abox shape with a lower sidewall 226 forming a bearing hole through whichthe lower end of the first pump shaft 211 a extends and a bearingportion for receiving the lower end of the second pump shaft 211 b, anda peripheral wall 227 extending upwardly from a peripheral edge of thelower sidewall 226. The upper side of the second housing 225 forms anopening through which the power transmission mechanism 240 can be placedinto the second housing 225.

[0199] The second housing 225 is connected to the first housing 221 insuch a manner as to have the opening sealed in a liquid tight manner bythe first sidewall 222 of the first housing 221, and form anaccommodation space of the power transmission mechanism 240 incooperation with the first sidewall 222 of the first housing 221.

[0200] In the thus arranged housing 220, the first sidewall 222 of thefirst housing 221 serves as a partition wall dividing the accommodationspace of the housing into a hydraulic pump accommodation chamber and apower transmission mechanism accommodation chamber. The partition wallthus defining the hydraulic pump accommodation chamber and the powertransmission mechanism accommodation chamber can effectively prevent anyforeign matters such as iron powder generated in the power transmissionmechanism 240 from intruding into the hydraulic pump accommodationchamber, and hence damaging piston units 212 a, 212 b, cylinder blocks213 a, 213 b, and/or other parts. In addition to this foreign mattersprevention measure, the first and second pump shafts 211 a and 211 b,which extend through the partition wall 222, may have circumferentialperipheries with seal rings thereon to more securely prevent theintrusion of the foreign matters.

[0201] Portions of the housing 220, through which the respective shafts211 a, 215 a and 215 b extend, are sealed by any suitable sealing meansin a liquid tight manner, thereby allowing the housing 220 to serve asthe hydraulic fluid tank 285.

[0202] The first sidewall 222 serving as the partition wall preferablyforms a hydraulic fluid communication hole 222 a for communicationbetween the hydraulic pump accommodation chamber and the powertransmission mechanism accommodation chamber with a filter 222 bprovided in the hole for preventing the intrusion of the foreign mattersinto the hydraulic pump accommodation chamber. The thus formed hydraulicfluid communication hole 222 a can omit the necessity of separatelyfeeding the lubricant to the power transmission mechanism 240, with theresult that the power transmission mechanism 240 can be lubricated withthe hydraulic fluid stored within the housing. This permits lowmanufacturing cost and ease of maintenance.

[0203] In this embodiment, the first and second angularly adjustableswash plates 214 a and 214 b are of a cradle type, as illustrated inFIG. 17. Therefore, when the partition wall 222 forms, on its sidefacing the hydraulic pumps 210 a, 210 b, spherical concave surfaces 222c respectively adapted to spherical convex surfaces 216 formed in therear sides of the swash plates 214 a and 214 b, which rear sides beingopposite to the surfaces facing the piston units 212 a and 212 b, thespherical concave surfaces 222 c can slidingly guide the sphericalconvex surfaces 216 of the swash plates 214 a and 214 b. The swashplates thus can securely rest on the spherical concave surfaces 222 c.Although FIG. 17 illustrates only the portion of the partition wall 222corresponding to the first angularly adjustable swash plate 214 a, it isa matter of course that the portion of the partition wall 222corresponding to the second angularly adjustable swash plate 214 b formsthe spherical concave surface 222 c.

[0204] In this embodiment, the first sidewall 222 of the first housing221 serves as the partition wall. Alternatively, a partitioning meansmay take various forms, as long as it can produce the same effect asdescribed above. For example, a separately prepared partition wall maybe mounted in a housing having a simple cylindrical box shape (see FIG.14).

[0205] Now, the description will be made for the center section 230. Asillustrated in FIG. 18, the center section 230 forms a first pair ofhydraulic passages 231 a for the first hydraulic pump communicating withthe first piston unit. The first pair of hydraulic passages 231 arespectively have first ends opening to the outside of the centersection 230 to form a first pair of inlet/outlet ports 232 a serving asconnection ports for connection with the first pair of hydraulic lines284 a extending between the first hydraulic motor and the center section230 (see FIG. 15).

[0206] Similarly, the center section 230 forms a second pair ofhydraulic passages 231 b for the second hydraulic pump communicatingwith the second piston unit. The second pair of hydraulic passages 231 brespectively have first ends forming a second pair of inlet/outlet ports232 b serving as connection ports for connection with the second pair ofhydraulic lines 284 b (see FIG. 15).

[0207] As described above, the common center section 230 thus forms allthe first and second pairs of inlet/outlet ports 232 a and 232 b servingas the connection ports for connection with the first and second pairsof hydraulic lines 284 a and 284 b. Whereby, the piping work between thehydraulic pumps 210 a and 210 b, and the hydraulic motors 282 a and 282b can be facilitated. The first and second pairs of inlet/outlet portsare more preferably formed in the same side of the center section 230,as illustrated in FIG. 18, thereby further facilitating the piping work.

[0208] The center section 230 also forms a common charging passage 233for feeding a pressurized hydraulic fluid to the first pair of hydrauliclines 284 a and the second pair of hydraulic lines 284 b. The chargingpassage 233 has a first end opening to the outside of the center section230 to form an inlet port for charging 234. In this embodiment, thefirst pump shaft 211 a, as illustrated in FIG. 16, has an extensionextending further from the upper end thereof to be located above thecenter section 230, thereby supporting a charge pump 250 via theextension, and connecting an outlet port 251 of the charge pump 250 tothe inlet port 234. The outlet port 251 of the charge pump alsocommunicates with a pressure relief line 253 having a charge reliefvalve 252 therein. The charge relief valve 252 is designed to adjust thehydraulic pressure in the charging passage 233 (see FIG. 15). Thepressure relief line 253 has a rear end communicating via a drain port235 formed in the center section 230 with the housing 220 serving alsoas the hydraulic fluid tank 285. The reference codes 255 and 256 inFIGS. 16 and 17 respectively represent an inlet port of the charge pump,and an inlet port communicating with the inlet port of the charge pumpand connected to the hydraulic fluid tank 285 through a suitableconduit.

[0209] On the other hand, the charging passage 233, as illustrated inFIG. 18, has a second end communicating with the first pair of hydraulicpassages 231 a and the second pair of hydraulic passages 231 b via checkvalves 261 a, 261 b, 261 c and 261 d so as to allow the pressurizedhydraulic fluid to be fed from the common charging passage 233 into alower pressure line of the first pair of hydraulic lines 284 a and alower pressure line of the second pairs of hydraulic lines 284 b, whilepreventing the pressurized hydraulic fluid from flowing in the reversedirection.

[0210] Bypass lines 262 a and 262 b having throttle valves are formedbetween at least one of the first pair of hydraulic passages 231 a andthe charging passage 233, and between at least one of the second pair ofhydraulic passages 231 b and the charging passage 233 (see FIGS. 15 and18).

[0211] The bypass lines 262 a and 262 b are designed to assure theneutralization of the hydraulic pumps 210 a and 210 b. Specifically,even if the swash plates 214 a and 214 b of the hydraulic pumps 210 aand 210 b tilt from the neutral positions by a small angle, there occursthe pressure difference between the first pair of hydraulic lines 284 a,and/or between the second pair of hydraulic lines 284 b. This pressuredifference causes the rotation of the hydraulic motors 282 a and 282 b.That is, even a slight amount of the displacement between the actualneutral positions and the predetermined design positions of the swashplates 214 a and 214 b due to assembling errors or the like causes anunintentional rotation of the hydraulic motors 282 a and 282 b. On thecontrary, the bypass lines 262 a and 262 b, as described above, allowthe pressurized hydraulic fluid to leak therethrough from the first pairof hydraulic lines 284 a and the second pair of hydraulic lines 284 b.Thus, the pressure difference between the pair of first hydraulic lines284 a and/or between the second pair of hydraulic lines 284 b caneffectively be limited, thereby effectively avoiding the unintentionalrotation of the hydraulic motors 282 a and 282 b, even for the swashplates 214 a and 214 b having the actual neutral position displaced fromthe design neutral position due to the assembling errors or the like.

[0212] In view of transmission efficiency between the hydraulic pumps210 a, 210 b and the hydraulic motors 282 a, 282 b, the leakage of thepressurized hydraulic fluid from the first and second pairs of hydrauliclines 284 a, 284 b through the bypass lines 262 a, 262 b is notpreferable. Therefore, the bypass lines 262 a, 262 b are preferablyprovided in portions from the charging passage 233 to one of the firstpair of hydraulic passages 231 a, and to one of the second pair ofhydraulic passages 231 b.

[0213] The check valves 261 a, 261 b, 261 c and 261 d are morepreferably provided with release means 262 to forcibly bring the firstpair of hydraulic passages 231 a into communication with one another,and the second pair of hydraulic passages 231 b into communication withone another, if an emergency arises, as illustrated in FIG. 18. Therelease means 262 are designed to easily move the vehicle, when thevehicle must forcibly be moved or the vehicle wheels must forcibly berotated by man power or the like due to the disorder of the power source280, the hydraulic pumps 210 a, 210 b or the like. Specifically, whenthe vehicle wheels connected to the hydraulic motors 282 a and 282 b areforcibly rotated with the first pair of hydraulic lines 284 a and/or thesecond pair of hydraulic lines 284 b lying in the closing state, thereoccurs the pressure difference between the first pair of hydraulic lines284 a, and between the second pair of hydraulic lines 284 b. As aresult, the vehicle is hardly moved, or the vehicle wheels are hardlyrotated. On the contrary, the release means can easily achieve thecommunications between the first pair of hydraulic passages 231 a, andbetween the second pair of hydraulic passages 231 b by mechanicallyreleasing all the check valves 261 a to 261 d. Whereby, the vehicle caneasily be moved by man power or the like.

[0214] As illustrated in FIG. 18, all the release means 263 arepreferably disposed in the same side of the center section 230, so thatthe link mechanism linking these release means 263 for operation of thesame can have a simplified structure.

[0215] The pump unit of this embodiment includes the charge pump 250 toforcibly feed the pressurized hydraulic fluid into the inlet port forcharging 234. As an alternative to the arrangement using the chargepump, the pump unit may have an arrangement where the inlet port 234 isconnected to the hydraulic fluid tank, thereby allowing the hydraulicfluid to spontaneously flow into the inlet port 234 when the pressure ina lower pressure line of the first pair of hydraulic lines 284 a and/orthe pressure in a lower pressure line of the second pair of hydrauliclines 284 b drops from a predetermined value.

Fourth Embodiment

[0216] Another embodiment of the pump unit according to the secondaspect of the present invention will be hereinafter described withreference to FIGS. 20 to 23. In the following description, correspondingor identical parts to those of the third embodiment have been given thesame reference characters or those with primes (′) to omit a detaileddescription thereof.

[0217]FIGS. 20 and 21 are respectively a longitudinal cross-sectionalside view, and a longitudinal cross-sectional front view of the pumpunit according to this embodiment. FIGS. 22 and 23 are respectivelycross sections taken along lines XXII-XXII and XXIII-XXIII in FIG. 20.

[0218] As illustrated in FIGS. 20 and 21, the pump unit 200′ of thisembodiment includes the first hydraulic pump 210 a′ and the secondhydraulic pump 210 b′, both of which are disposed parallel to oneanother along the vehicle longitudinal direction, and the first andsecond hydraulic pumps 210 a′ and 210 b′ respectively having theangularly adjustable swash plates 214 a′ and 214 b′ of trunnion type.

[0219] As illustrated in FIGS. 21 and 23, the first control shaft 215 aand the second control shaft 215 b extend away from one another alongthe vehicle width direction, in the same manner as those of theaforementioned embodiments.

[0220] As illustrated in FIG. 22, both first and second pairs ofinlet/outlet ports 232 a and 232 b are formed in the same side of thecenter section 230′. The pump unit of this embodiment is also arrangedso that the pressurized hydraulic fluid can be fed into the first pairof hydraulic passages 231 a and the second pair of hydraulic passages231 b via the inlet port for charging 234 and the charging passage 233communicating with the port 234.

[0221] The pump unit 200′ having the first and second hydraulic pumps210 a′ and 210 b′ arranged parallel to one another along thelongitudinal direction has the first control shaft 215 a displaced fromthe second control shaft 215 b with respect to the vehicle longitudinaldirection, as illustrated in FIG. 23. This displacement can be easilycompensated by using arms or other suitable linking means.

[0222] The pump unit 200′ having the above arrangement also produces thesame effects as those of the third embodiment.

Fifth Embodiment

[0223] One embodiment of the pump unit according to the third aspect ofthe present invention will be hereinafter described with reference toFIGS. 24 to 28. FIG. 24 is a hydraulic circuit diagram of the vehicle towhich a pump unit 300 of this embodiment is applied. FIG. 25 is alongitudinal cross-sectional front view of the pump unit and itsperiphery. FIGS. 26 to 28 are respectively cross sections taken alonglines XXVI-XXVI, XXVII-XXVII and XXVIII-XXVIII in FIG. 25.

[0224] As illustrated in FIGS. 24 to 26, the pump unit 300 is adapted tobe used in a vehicle having right and left drive wheels 383 a and 383 bto which first and second hydraulic motors 382 a and 382 b arerespectively connected, and includes a first hydraulic pump 310 a and asecond hydraulic pump 310 b respectively connected to the first andsecond hydraulic motors 382 a and 382 b via a first pair of hydrauliclines 384 a and a second pair of hydraulic lines 384 b, and a commonhousing 320 for accommodating these hydraulic pumps 310 a and 310 b.

[0225] The connection form between the right and left drive wheels 383 aand 383 b, and the first and second hydraulic motors 382 a and 382 bmeant in this embodiment includes the direct connection of the drivewheels respectively to those hydraulic motors, and also an operativeconnection of the drive wheels respectively to the hydraulic motors viaa suitable power transmission mechanism. In FIG. 24, the reference codes380, 381 and 385 respectively represent a power source, a cooling fanand a hydraulic fluid tank.

[0226] As illustrated in FIGS. 25, 26 and 28, the first hydraulic pump310 a and the second hydraulic pump 310 b are axial piston pumps of avariable displacement type, and respectively include a first pump shaft311 a and a second pump shaft 311 b that have vertical axes and aredisposed parallel to one another in the vehicle width direction withinthe housing 320, a first piston unit 312 a and a second piston unit 312b that are reciprocatingly movable according to the rotation of the pumpshafts, a first cylinder block 313 a and a second cylinder block 313 bthat reciprocably support the piston units, a first angularly adjustableswash plate 314 a and a second angularly adjustable swash plate 314 bthat regulate the stroke length of the piston units by varying theirtilting angles to vary the input/output flow rates of the piston units,and a first control shaft 315 a and a second control shaft 315 b thatcontrol the tilting angles of these swash plates.

[0227] The pump unit of this embodiment is of a vertical type with thefirst and second pump shafts 311 a and 311 b having the vertical axes.However, the second aspect of the present invention is not necessarilylimited to this arrangement. It is a matter of course that the pump unit300 can be of a horizontal type with the first and second pump shafts311 a and 311 b having the horizontal axes.

[0228] As best illustrated in FIG. 25, the first and second angularlyadjustable swash plates 314 a and 314 b of this embodiment are of acradle type.

[0229] As illustrated in FIGS. 25 and 28, the first control shaft 315 aand the second control shaft 315 b extend away from one another in thevehicle width direction to respectively have oppositely positioned outerends, and inner ends extending into the housing 320 to be respectivelyconnected to arms 316 a and 316 b and hence the first and second swashplates 314 a and 314 b. The pump unit 300 with the thus arranged firstand second control shafts 315 a and 315 b is advantageous when installedon the vehicle having push-pull control levers 198 a and 198 b asillustrated in FIG. 1, since the first and second control shafts 315 aand 315 b can have the rotating shaft centers disposed parallel to thelongitudinal axis of the control levers, thereby achieving thesimplification of a link mechanism between these control shafts and thecontrol levers.

[0230] The first control shaft 315 a and the second control shaft 315 bare more preferably located at the same position with respect to thevehicle longitudinal direction, as illustrated in FIG. 25. The thusarranged first and second control shafts 315 a and 315 b can be alignedwith the control levers in the vehicle width direction, therebyachieving a more simplified structure of the link mechanism.

[0231] The pump unit 300 further includes a common center section 330that supports the first and second hydraulic pumps 310 a and 310 b, anda power transmission mechanism 340 that is accommodated within thehousing 320 to operatively connect the first and second hydraulic pumpshafts 311 a and 311 b together.

[0232] The pump unit 300 with the power transmission mechanism 340permits the simultaneous rotation of both pump shafts 311 a and 311 bonly by connecting the power source to either one of the first andsecond pump shafts 311 a and 311 b, or to the first pump shaft 311 a inthis embodiment, thereby achieving the simplified structure for thepower transmission from the power source to the pump unit 300. In thisembodiment, the power transmission mechanism 340 is in the form of agear transmission device that includes a first gear 340 a non-rotatablysupported on the lower side of the first pump shaft 311 a, and a secondgear 340 b non-rotatably supported on the lower side of the second pumpshaft 311 b in meshed engagement with the first gear 340 a. Instead ofthe gear transmission device, any conventional power transmissionmechanisms such as chain and belt may be used.

[0233] The housing 320, as illustrated in FIGS. 25 and 26, includes afirst housing 321 for accommodating the first and second hydraulic pumps310 a and 310 b, and a second housing 325 for accommodating the powertransmission mechanism 340.

[0234] The first housing 321 has a box shape with a first side wall 322disposed in the upper or lower side of the pump shafts 311 a and 311 balong the longitudinal direction thereof, or in this embodiment in thelower side of the pump shafts 311 a and 311 b, which will be hereinafterreferred to simply as the lower side, and a peripheral wall 323extending from a peripheral edge of the first sidewall 322 to theopposite side of the pump shafts 311 a and 311 b along the longitudinaldirection thereof (i.e., the upper side of the pump shafts 311 a and 311b in this embodiment, which will be referred to simply as the upperside). The first sidewall 322 forms bearing holes through which thefirst and second pump shafts 311 a and 311 b respectively extend. Theupper side of the first housing 321 has an end surface forming anopening through which the first and second hydraulic pumps 310 a and 310b can be placed into the first housing 321. The opening of the firsthousing 321 is sealed by the center section 330 in a liquid tightmanner. That is, the center section 330 of this embodiment constitutes apart of the first housing 321. The first and second control shafts 315 aand 315 b extend away from one another in the vehicle width direction torespectively have outer ends protruding from the peripheral wall 323 ofthe first housing 321.

[0235] The second housing 325 is disposed in the lower side, and has abox shape with a lower sidewall 326 forming a bearing hole through whichthe lower end of the first pump shaft 311 a extends and a bearingportion for receiving the lower end of the second pump shaft 311 b, anda peripheral wall 327 extending upwardly from a peripheral edge of thelower sidewall 326. The upper side of the second housing 325 forms anopening through which the power transmission mechanism 340 can be placedinto the second housing 325.

[0236] The second housing 325 is connected to the first housing 321 insuch a manner as to have the opening sealed in a liquid tight manner bythe first sidewall 322 of the first housing 321, and form anaccommodation space of the power transmission mechanism 340 incooperation with the first sidewall 322 of the first housing 321.

[0237] In the thus arranged housing 320, the first sidewall 322 of thefirst housing 321 serves as a partition wall dividing the accommodationspace of the housing into a hydraulic pump accommodation chamber and apower transmission mechanism accommodation chamber. The partition wallthus defining the hydraulic pump accommodation chamber and the powertransmission mechanism accommodation chamber can effectively prevent anyforeign matters such as iron powder generated in the power transmissionmechanism 340 from intruding into the hydraulic pump accommodationchamber, and hence damaging piston units 312 a, 312 b, cylinder blocks313 a, 313 b, and/or other parts. In addition to this foreign mattersprevention measure, the first and second pump shafts 311 a and 311 b,which extend through the partition wall 322, may have circumferentialperipheries with seal rings thereon to more securely prevent theintrusion of the foreign matters.

[0238] Portions of the housing 320, through which the respective shafts311 a, 315 a and 315 b extend, are sealed by any suitable sealing meansin a liquid tight manner, thereby allowing the housing 320 to serve asthe hydraulic fluid tank 385.

[0239] The first sidewall 322 serving as the partition wall preferablyforms a hydraulic fluid communication hole 322 a for communicationbetween the hydraulic pump accommodation chamber and the powertransmission mechanism accommodation chamber with a filter 322 bprovided in the hole for preventing the intrusion of the foreign mattersinto the hydraulic pump accommodation chamber. The thus formed hydraulicfluid communication hole 322 a can omit the necessity of separatelyfeeding the lubricant to the power transmission mechanism 340, with theresult that the power transmission mechanism 340 can be lubricated withthe hydraulic fluid stored within the housing. This permits lowmanufacturing cost and ease of maintenance.

[0240] In this embodiment, the first and second angularly adjustableswash plates 314 a and 314 b are of a cradle type, as illustrated inFIG. 26. Therefore, when the partition wall 322 forms, on its sidefacing the hydraulic pumps 310 a, 310 b, spherical concave surfaces 322c respectively adapted to spherical convex surfaces 316 formed in therear sides of the swash plates 314 a and 314 b, which rear sides beingopposite to the surfaces facing the piston units 312 a and 312 b, thespherical concave surfaces 322 c can slidingly guide the sphericalconvex surfaces 316 of the swash plates 314 a and 314 b. The swashplates thus can securely rest on the spherical concave surfaces 322 c.Although FIG. 26 illustrates only the portion of the partition wall 322corresponding to the first angularly adjustable swash plate 314 a, it isa matter of course that the portion of the partition wall 322corresponding to the second angularly adjustable swash plate 314 b formsthe spherical concave surface 322 c.

[0241] In this embodiment, the first sidewall 322 of the first housing321 serves as the partition wall. Alternatively, a partitioning meansmay take various forms, as long as it can produce the same effect. Forexample, a separately prepared partition wall may be mounted in ahousing having a simple cylindrical box shape (see FIG. 14).

[0242] Now, the description will be made for the center section 330. Asillustrated in FIG. 27, the center section 330 forms a first pair ofhydraulic passages 331 a for the first hydraulic pump communicating withthe first piston unit. The first pair of hydraulic passages 331 arespectively have first ends opening to the outside of the centersection 330 to form a first pair of inlet/outlet ports 332 a serving asconnection ports for connection with the first pair of hydraulic lines384 a extending between the first hydraulic motor and the center section330 (see FIG. 24).

[0243] Similarly, the center section 330 forms a second pair ofhydraulic passages 331 b for the second hydraulic pump communicatingwith the second piston unit.

[0244] The second pair of hydraulic passages 331 b respectively havefirst ends forming a second pair of inlet/outlet ports 332 b serving asconnection ports for connection with the second pair of hydraulic lines384 b (see FIG. 24).

[0245] As described above, the common center section 330 thus forms allthe first and second pairs of inlet/outlet ports 332 a and 332 b servingas the connection ports for connection with the first and second pairsof hydraulic lines 384 a and 384 b. Whereby, the conduit work betweenthe hydraulic pumps 310 a and 310 b, and the hydraulic motors 382 a and382 b can be facilitated. The first and second pairs of inlet/outletports are more preferably formed in the same side of the center section330, as illustrated in FIG. 27, thereby further facilitating the conduitwork.

[0246] The center section 330 also forms a common-charging passage 333for feeding a pressurized hydraulic fluid to the first pair of hydrauliclines 384 a and the second pair of hydraulic lines 384 b. The chargingpassage 333 has a first end opening to the outside of the center section330 to form an inlet port for charging 334. In this embodiment, thefirst pump shaft 311 a, as illustrated in FIG. 25, has an extensionextending further from the upper end thereof to be located above thecenter section 330, thereby supporting a charge pump 350 via theextension, and connecting an outlet port 351 of the charge pump 350 tothe inlet port 334. The outlet port 351 of the charge pump alsocommunicates with a pressure relief line 353 having a charge reliefvalve 352 therein. The charge relief valve 352 is designed to adjust thehydraulic pressure in the charging passage 333 (see FIG. 24). Thepressure relief line 353 has a rear end communicating via a drain port335 formed in the center section 330 with the housing 320 serving alsoas the hydraulic fluid tank 385. The reference codes 355 and 356 inFIGS. 25 and 26 respectively represent an inlet port of the charge pump,and an inlet port communicating with the inlet port of the charge pumpand connected to the hydraulic fluid tank 385 through a suitableconduit.

[0247] On the other hand, the charging passage 333, as illustrated inFIG. 27, has a second end communicating with the first pair of hydraulicpassages 331 a and the second pair of hydraulic passages 331 b via checkvalves 361 a, 361 b, 361 c and 361 d so as to allow the pressurizedhydraulic fluid to be fed from the common charging passage 333 into alower pressure line of the first pair of hydraulic lines 384 a and alower pressure line of the second pair of hydraulic lines 384 b, whilepreventing the pressurized hydraulic fluid from flowing in the reversedirection.

[0248] Bypass lines 362 a and 362 b having throttle valves are formedbetween at least one of the first pair of hydraulic passages 331 a andthe charging passage 333, and between at least one of the second pair ofhydraulic passages 331 b and the charging passage 333 (see FIGS. 24 and27).

[0249] The bypass lines 362 a and 362 b are designed to assure theneutralization of the hydraulic pumps 310 a and 310 b. Specifically,even if the swash plates 314 a and 314 b of the hydraulic pumps 310 aand 310 b tilt from the neutral positions by a small angle, there occursthe pressure difference between the first pair of hydraulic lines 384 a,and/or between the second pair of hydraulic lines 384 b. This pressuredifference causes the rotation of the hydraulic motors 383 a and 383 b.That is, even a slight amount of the displacement between the actualneutral positions and the predetermined design positions of the swashplates 314 a and 314 b due to assembling errors or the like causes anunintentional rotation of the hydraulic motors 383 a and 383 b. On thecontrary, the bypass lines 362 a and 362 b, as described above, allowthe pressurized hydraulic fluid to leak therethrough from the first pairof hydraulic lines 384 a and the second pair of hydraulic lines 384 b.Thus, the pressure difference between the pair of first hydraulic lines384 a and/or between the second pair of hydraulic lines 384 b caneffectively be limited, thereby effectively avoiding the unintentionalrotation of the hydraulic motors 382 a and 382 b, even for the swashplates 314 a and 314 b having the actual neutral position displaced fromthe design neutral position due to the assembling errors or the like.

[0250] In view of transmission efficiency between the hydraulic pumps310 a, 310 b and the hydraulic motors 382 a, 382 b, the leakage of thepressurized hydraulic fluid from the first and second pairs of hydrauliclines 384 a, 384 b through the bypass lines 362 a, 362 b is notpreferable. Therefore, the bypass lines 362 a, 362 b are preferablyprovided in portions from the charging passage 333 to one of the firstpair of hydraulic passages 331 a, and to one of the second pair ofhydraulic passages 331 b.

[0251] The check valves 361 a, 361 b, 361 c and 361 d are morepreferably provided with release means 362 to forcibly bring the firstpair of hydraulic passages 331 into communication with one another, andthe second pair of hydraulic passages 331 b into communication with oneanother, if an emergency arises, as illustrated in FIG. 27. The releasemeans 362 are designed to easily move the vehicle, when the vehicle mustforcibly be moved or the vehicle wheels must forcibly be rotated by manpower or the like due to the disorder of the power source 380, thehydraulic pumps 310 a, 310 b or the like. Specifically, when the vehiclewheels connected to the hydraulic motors 382 a and 382 b are forciblyrotated with the first pair of hydraulic lines 384 a and/or the secondpair of hydraulic lines 384 b lying in the closing state, there occursthe pressure difference between the first pair of hydraulic lines 331 a,and between the second pair of hydraulic lines 331 b. As a result, thevehicle is hardly moved, or the vehicle wheels are hardly rotated. Onthe contrary, the release means can easily achieve the communicationsbetween the first pair of hydraulic lines 384 a, and between the secondpair of hydraulic lines 384 b by mechanically releasing all the checkvalves 361 a to 361 d. Whereby, the vehicle can easily be moved by manpower or the like.

[0252] As illustrated in FIG. 27, all the release means 363 arepreferably disposed in the same side of the center section 330, so thatthe link mechanism for linking these release means 363 can have asimplified structure.

[0253] The pump unit of this embodiment includes the charge pump 350 toforcibly feed the pressurized hydraulic fluid into the inlet port forcharging 334. As an alternative to the arrangement using the chargepump, the pump unit may have an arrangement where the inlet port 334 isconnected to the hydraulic fluid tank, thereby allowing the hydraulicfluid to spontaneously flow into the inlet port 334 when the pressure ina lower pressure line of the first pair of hydraulic lines 384 a and/orthe pressure in a lower pressure line of the second pair of hydrauliclines 384 b drops from a predetermined value.

Sixth Embodiment

[0254] Another embodiment of the pump unit according to the third aspectof the present invention will be hereinafter described with reference toFIGS. 29 to 32. In the following description, corresponding or identicalparts to those of the fifth embodiment have been given the samereference characters or those with primes (′) to omit a detaileddescription thereof.

[0255]FIGS. 29 and 30 are respectively a longitudinal cross-sectionalside view, and a longitudinal cross-sectional front view of the pumpunit according to this embodiment. FIGS. 31 and 32 are respectivelycross sections taken along lines XXXI-XXXI, and XXXII-XXXII in FIG. 29.

[0256] As illustrated in FIGS. 29 and 30, the pump unit 300′ of thisembodiment includes the first hydraulic pump 310 a′ and the secondhydraulic pump 310 b′, both of which are disposed parallel to oneanother along the vehicle longitudinal direction, and the first andsecond hydraulic pumps 310 a′ and 310 b′ respectively having theangularly adjustable swash plates 314 a′ and 314 b′ of trunnion type.

[0257] As illustrated in FIGS. 30 and 32, the first control shaft 315 aand the second control shaft 315 b extend away from one another alongthe vehicle width direction, in the same manner as those of theaforementioned embodiments.

[0258] As illustrated in FIG. 31, both first and second pairs ofinlet/outlet ports 332 a and 332 b are formed in the same side of thecenter section 330′. The pump unit of this embodiment is also arrangedso that the pressurized hydraulic fluid can be fed into the first pairof hydraulic passages 331 a and the second pair of hydraulic passages331 b via the inlet port for charging 334 and the charging passage 333communicating with the port 334.

[0259] The pump unit 300′ having the first and second hydraulic pumps310 a′ and 310 b′ arranged parallel to one another along thelongitudinal direction has the first control shaft 315 a displaced fromthe second control shaft 315 b with respect to the vehicle longitudinaldirection, as illustrated in FIG. 32. This displacement can be easilycompensated by using arms or other suitable linking means.

[0260] The pump unit 300′ having the above arrangement also produces thesame effects as those of the fifth embodiment.

Seventh Embodiment

[0261] One embodiment of the pump unit according to the fourth aspect ofthe present invention will be hereinafter described with reference tothe accompanying drawings. FIG. 33 is a hydraulic circuit diagram of thevehicle to which a pump unit 400 of this embodiment is applied. FIG. 34is a longitudinal cross-sectional side view of the pump unit and itsperiphery. FIGS. 35 and 36 are respectively cross sections taken alonglines XXXV-XXXV, and XXXVI-XXXVI.

[0262] As illustrated in FIGS. 33 to 35, the pump unit 400 is adapted tobe used in a vehicle having right and left drive wheels 483 a and 483 bto which first and second hydraulic motors 482 a and 482 b arerespectively connected, and includes a first hydraulic pump 410 a and asecond hydraulic pump 410 b respectively connected to the first andsecond hydraulic motors 482 a and 482 b via a first pair of hydrauliclines 484 a and a second pair of hydraulic lines 484 b, and a commonhousing 420 for accommodating these hydraulic pumps 410 a and 410 b.

[0263] The connection form between the right and left drive wheels 483 aand 483 b, and the first and second hydraulic motors 482 a and 482 bmeant in this embodiment includes the direct connection of the drivewheels respectively to those hydraulic motors, and also an operativeconnection of the drive wheels respectively to the hydraulic motors viaa suitable power transmission mechanism. In FIG. 33, the reference codes480, 481 and 486 respectively represent a power source, a cooling fanand a filter.

[0264] As illustrated in FIGS. 34 to 36, the first hydraulic pump 410 aand the second hydraulic pump 410 b are axial piston pumps of a variabledisplacement type, and respectively include a first pump shaft 411 a anda second pump shaft 411 b that have vertical axes and are disposedparallel to one another in the vehicle width direction within thehousing 420, a first piston unit 412 a and a second piston unit 412 bthat are reciprocatingly movable according to the rotation of the pumpshafts, a first cylinder block 413 a and a second cylinder block 413 bthat reciprocably support the piston units, a first angularly adjustableswash plate 414 a and a second angularly adjustable swash plate 414 bthat regulate the stroke length of the piston units by varying theirtilting angles to vary the input/output flow rates of the piston units,and a first control shaft 415 a and a second control shaft 415 b thatcontrol the tilting angles of these swash plates.

[0265] The pump unit of this embodiment is of a vertical type with thefirst and second pump shafts 411 a and 411 b having the vertical axes.However, the second aspect of the present invention is not necessarilylimited to this arrangement. It is a matter of course that the pump unit400 can be of a horizontal type with the first and second pump shafts411 a and 411 b having the horizontal axes.

[0266] As best illustrated in FIG. 34, the first and second angularlyadjustable swash plates 414 a and 414 b of this embodiment are of acradle type.

[0267] As illustrated in FIGS. 35 and 36, the first control shaft 415 aand the second control shaft 415 b extend away from one another in thevehicle width direction to respectively have oppositely positioned outerends, and inner ends extending into the housing 420 to be respectivelyconnected to arms 416 a and 416 b and hence the first and second swashplates 414 a and 414 b. The pump unit 400 with the thus arranged firstand second control shafts 415 a and 415 b is advantageous when installedon the vehicle having push-pull control levers 198 a and 198 b asillustrated in FIG. 1, since the first and second control shafts 415 aand 415 b can have the rotating shaft centers disposed parallel to thelongitudinal axis of the control levers, thereby achieving thesimplification of a link mechanism between these control shafts and thecontrol levers.

[0268] The first control shaft 415 a and the second control shaft 415 bare more preferably located at substantially the same position withrespect to the vehicle longitudinal direction, as illustrated in FIG.34. The thus arranged first and second control shafts 415 a and 415 bcan be aligned with the control levers in the vehicle width direction,thereby achieving a more simplified structure of the link mechanism.

[0269] The pump unit 400 further includes a common center section 430that supports the first and second hydraulic pumps 410 a and 410 b, anda power transmission mechanism 440 that is accommodated within thehousing 420 to operatively connect the first and second hydraulic pumpshafts 411 a and 411 b together.

[0270] The pump unit 400 with the power transmission mechanism 440permits the simultaneous rotation of both pump shafts 411 a and 411 bonly by connecting the power source to either one of the first andsecond pump shafts 411 a and 411 b, or to the first pump shaft 411 a inthis embodiment, thereby achieving the simplified structure for thepower transmission from the power source to the pump unit 400. In thisembodiment, the power transmission mechanism 440 is in the form of agear transmission device that includes a first gear 440 a non-rotatablysupported on the lower side of the first pump shaft 411 a, and a secondgear 440 b non-rotatably supported on the lower side of the second pumpshaft 411 b in meshed engagement with the first gear 440 a. Instead ofthe gear transmission device, any conventional power transmissionmechanisms such as chain and belt may be used.

[0271] The housing 420, as illustrated in FIGS. 34 and 35, includes afirst housing 421 for accommodating the first and second hydraulic pumps410 a and 410 b, and a second housing 422 for accommodating the powertransmission mechanism 440.

[0272] The first housing 421 has a box shape with a first sidewall 421 adisposed in the upper or lower side of the pump shafts 411 a and 411 balong the longitudinal direction thereof, or in this embodiment in thelower side of the pump shafts 411 a and 411 b, which will be hereinafterreferred to simply as the lower side, and a peripheral wall 421 bextending from a peripheral edge of the first sidewall 421 a to theopposite side of the pump shafts 411 a and 411 b along the longitudinaldirection thereof (i.e., the upper side of the pump shafts 411 a and 411b in this embodiment, which will be referred to simply as the upperside). The first sidewall 421 a forms bearing holes through which thefirst and second pump shafts 411 a and 411 b respectively extend. Theupper side of the first housing 421 has an end surface forming anopening through which the first and second hydraulic pumps 410 a and 410b can be placed into the first housing 421. The opening of the firsthousing 421 is sealed by the center section 430 in a liquid tightmanner. That is, the center section 430 of this embodiment constitutes apart of the first housing 421. The first and second control shafts 415 aand 415 b extend away from one another in the vehicle width direction torespectively have outer ends protruding from the peripheral wall 421 bof the first housing 421.

[0273] The second housing 422 is disposed in the lower side, and has abox shape with a lower sidewall 422 a forming a bearing hole throughwhich the lower end of the first pump shaft 411 a extends and a bearingportion for receiving the lower end of the second pump shaft 411 b, anda peripheral wall 422 b extending upwardly from a peripheral edge of thelower sidewall 422 a. The upper side of the second housing 422 forms anopening through which the power transmission mechanism 440 can be placedinto the second housing 422.

[0274] The second housing 422 is connected to the first housing 421 insuch a manner as to have the opening sealed in a liquid tight manner bythe first sidewall 421 a of the first housing 421, and form anaccommodation space of the power transmission mechanism 440 incooperation with the first sidewall 421 a of the first housing 421.

[0275] In the thus arranged housing 420, the first sidewall 421 a of thefirst housing 421 serves as a partition wall dividing the accommodationspace of the housing into a hydraulic pump accommodation chamber and apower transmission mechanism accommodation chamber. The partition wallthus defining the hydraulic pump accommodation chamber and the powertransmission mechanism accommodation chamber can effectively prevent anyforeign matters such as iron powder generated in the power transmissionmechanism 440 from intruding into the hydraulic pump accommodationchamber, and hence damaging piston units 412 a, 412 b, cylinder blocks413 a, 413 b, and/or other parts. In addition to this foreign mattersprevention measure, the first and second pump shafts 411 a and 411 b,which extend through the partition wall 421 a, may have circumferentialperipheries with seal rings thereon to more securely prevent theintrusion of the foreign matters.

[0276] Portions of the housing 420, through which the respective shafts411 a, 415 a and 415 b extend, are sealed by any suitable sealing meansin a liquid tight manner, thereby allowing the housing 420 to serve asthe hydraulic fluid tank.

[0277] The first sidewall 421 a serving as the partition wall preferablyforms a hydraulic fluid communication hole 423 a for communicationbetween the hydraulic pump accommodation chamber and the powertransmission mechanism accommodation chamber with a filter 423 bprovided in the hole for preventing the intrusion of the foreign mattersinto the hydraulic pump accommodation chamber. The thus formed hydraulicfluid communication hole 423 a can omit the necessity of separatelyfeeding the lubricant to the power transmission mechanism 440, with theresult that the power transmission mechanism 440 can be lubricated withthe hydraulic fluid stored within the housing. This permits lowmanufacturing cost and ease of maintenance.

[0278] In this embodiment, the first and second angularly adjustableswash plates 414 a and 414 b are of a cradle type, as illustrated inFIG. 34. Therefore, when the partition wall 421 a forms, on its sidefacing the hydraulic pumps 410 a, 410 b, spherical concave surfaces 424respectively adapted to spherical convex surfaces 419 formed in the rearsides of the swash plates 414 a and 414 b, which rear sides beingopposite to the surfaces facing the piston units 412 a and 412 b, thespherical concave surfaces 424 can slidingly guide the spherical convexsurfaces 419 of the swash plates 414 a and 414 b. The swash plates thuscan securely rest on the spherical concave surfaces 424. Although FIG.34 illustrates only the portion of the partition wall 421 acorresponding to the first angularly adjustable swash plate 414 a, it isa matter of course that the portion of the partition wall 421 acorresponding to the second angularly adjustable swash plate 414 b formsthe spherical concave surface 424.

[0279] In this embodiment, the first sidewall 421 a of the first housing421 serves as the partition wall. Alternatively, a partitioning meansmay take various forms, as long as it can produce the same effect asdescribed above. For example, a separately prepared partition wall maybe mounted in a housing having a simple cylindrical box shape (see FIG.14).

[0280] Now, the description will be made for the center section 430.FIG. 37 is an enlarged view of a portion XXXVII in FIG. 34. FIGS. 38 and39 are respectively cross sections taken along lines XXXVIII-XXXVIII,and XXXIX-XXXIX in FIG. 37. FIG. 40 is a cross section taken along linesXXXX-XXXX in FIG. 39.

[0281] As best illustrated in FIG. 39, the center section 430 forms afirst pair of hydraulic passages 431 a respectively having first endscommunicating with the first piston unit and second ends opening to theoutside of the center section 430 to form a first pair of inlet/outletports 432 a serving as connection ports for connection with the firstpair of hydraulic lines 484 a (see FIG. 33).

[0282] The center section 430 also forms a second pair of hydraulicpassages 431 b having first ends communicating with the second pistonunit and second ends opening to the outside of the center section 430 toform a second pair of inlet/outlet ports 432 b serving as connectionports for connection with the second pair of hydraulic lines 484 b (seeFIG. 33).

[0283] As described above, the common center section 430 thus forms allthe first and second pairs of inlet/outlet ports 432 a and 432 b servingas the connection ports for connection with the first and second pairsof hydraulic lines 484 a and 484 b. Whereby, the piping work between thehydraulic pumps 410 a and 410 b, and the hydraulic motors 482 a and 482b can be facilitated. The first and second pairs of inlet/outlet ports432 a and 432 b are more preferably formed in the same side of thecenter section 430, as illustrated in FIGS. 39 and 40, thereby furtherfacilitating the piping work.

[0284] The center section 430, as illustrated in FIGS. 37 to 40, alsoforms a common charging passage 433 having a first end opening to theoutside of the center section 430 to form an inlet port for charging 434serving as an inlet for the hydraulic fluid to be replenished, and asecond end communicating with the first pair of hydraulic passages 431 aand the second pair of hydraulic passages 431 b via check valves 461 a,461 b, 461 c and 461 d.

[0285] In this embodiment, the first pump shaft 411 a, as bestillustrated in FIGS. 34 and 37, has an extension extending upwards fromthe upper end thereof to be located above the center section 430,thereby supporting a charge pump 450 via the extension. The charge pump450 has an upper surface with a cartridge filter 486 detachably mountedthereto, through which the pressurized hydraulic fluid is fed from anoutlet port 451 of the charge pump 450 to the inlet port for charging434.

[0286] The cartridge filter 486 can be provided at the suction side ofthe charge pump 450.

[0287] The charging passage 433 is connected via a relief valve 452 to apressure relief line 453 communicating with the housing. The reliefvalve 452 regulates the hydraulic pressure of the charging passage 433(see FIGS. 33 and 40).

[0288] In this embodiment, the pressure relief line 453 is formed in acharge pump casing 459 mounted on the upper surface of the centersection 430 to be communicated with the first housing 421 via a drainport 435 formed in the center section 430. However, the presentinvention is not necessarily limited to this arrangement. That is, thepressure relief line 453 can be formed in the center section 430.

[0289] Reference codes 455, and 456 in FIGS. 34, 35 and 38 respectivelyrepresent an inlet port of the charge pump, and an inlet portcommunicating with the inlet port 455 of the charge pump.

[0290] As described above, the charging passage 433 has the second endcommunicating with the first pair of hydraulic passages 431 a and thesecond pair of hydraulic passages 431 b via the check valves 461 a, 461b, 461 c and 461 d so as to allow the pressurized hydraulic fluid to befed from the common charging passage 433 into a lower pressure line ofthe first pair of hydraulic lines 484 a and a lower pressure line of thesecond pairs of hydraulic lines 484 b, while preventing the pressurizedhydraulic fluid from flowing in the reverse direction.

[0291] Further, bypass lines 462 a and 462 b having throttle valves areformed between at least one of the first pair of hydraulic passages 431a and the charging passage 433, and between at least one of the secondpair of hydraulic passages 431 b and the charging passage 433 (see FIGS.33 and 39).

[0292] The bypass lines 462 a and 462 b are designed to assure theneutralization of the hydraulic pumps 410 a and 410 b. Specifically,even if the swash plates 414 a and 414 b of the hydraulic pumps 410 aand 410 b tilt from the neutral positions by a small angle, there occursthe pressure difference between the first pair of hydraulic lines 484 a,and/or between the second pair of hydraulic lines 484 b. This pressuredifference causes the rotation of the hydraulic motors 482 a and 482 b.That is, even a slight amount of the displacement between the actualneutral positions and the predetermined design positions of the swashplates 414 a and 414 b due to assembling errors or the like causes anunintentional rotation of the hydraulic motors 482 a and 482 b. On thecontrary, the bypass lines 462 a and 462 b, as described above, allowthe pressurized hydraulic fluid to leak therethrough from the first pairof hydraulic lines 484 a and the second pair of hydraulic lines 484 b.Thus, the pressure difference between the pair of first hydraulic lines484 a and/or between the second pair of hydraulic lines 484 b caneffectively be limited, thereby effectively avoiding the unintentionalrotation of the hydraulic motors 482 a and 482 b, even for the swashplates 414 a and 414 b having the actual neutral position displaced fromthe design neutral position due to the assembling errors or the like.

[0293] In view of transmission efficiency between the hydraulic pumps410 a, 410 b and the hydraulic motors 482 a, 482 b, the leakage of thepressurized hydraulic fluid from the first and second pairs of hydrauliclines 484 a, 484 b through the bypass lines 462 a, 462 b is notpreferable. Therefore, the bypass lines 462 a, 462 b are preferablyprovided in portions from the charging passage 433 to one of the firstpair of hydraulic passages 431 a, and to one of the second pair ofhydraulic passages 431 b.

[0294] The check valves 461 a, 461 b, 461 c and 461 d are morepreferably provided with release means 462 to forcibly bring the firstpair of hydraulic passages 431 a into communication with one another,and the second pair of hydraulic passages 431 b into communication withone another, if an emergency arises, as illustrated in FIG. 36. Therelease means 462 are designed to easily move the vehicle, when thevehicle must forcibly be moved or the vehicle wheels must forcibly berotated by man power or the like due to the disorder of the power source480, the hydraulic pumps 410 a, 410 b or the like. Specifically, whenthe vehicle wheels connected to the hydraulic motors 482 a and 482 b areforcibly rotated with the first pair of hydraulic lines 484 a and/or thesecond pair of hydraulic lines 484 b lying in the closing state, thereoccurs the pressure difference between the first pair of hydraulic lines484 a, and between the second pair of hydraulic lines 484 b. As aresult, the vehicle is hardly moved, or the vehicle wheels are hardlyrotated. On the contrary, the release means can easily achieve thecommunications between the first pair of hydraulic passages 431 a, andbetween the second pair of hydraulic passages 431 b by mechanicallyreleasing all the check valves 461 a to 461 d. Whereby, the vehicle caneasily be moved by man power or the like.

[0295] As illustrated in FIG. 39, all the release means 463 arepreferably disposed in the same side of the center section 430, so thatthe link mechanism linking these release means 463 for operation of thesame can have a simplified structure.

[0296] As described above, the pump unit 400 of this embodiment includesthe first and second hydraulic pumps 410 a and 410 b, the center section430 and the housing 420, all of which are integrally connected togetherto constitute a single unit 400 a. Accordingly, both first and secondhydraulic pumps 410 a and 410 b can be installed on the vehicle only bymounting the single unit 400 a on the vehicle, thereby achieving animproved efficiency in assembling the vehicle.

[0297] The pump unit 400 of this embodiment also includes a reservoirtank 485 supportingly connected to the single unit 400 a, as illustratedin FIGS. 34 to 36. The reservoir tank 485 is designed to reserve thehydraulic fluid to be replenished to the first pair of hydraulicpassages 431 a and the second pair of hydraulic passages 431 b. In thisembodiment, the reservoir tank 485 has right and left sides respectivelyforming mounting ribs 485 a, through which the reservoir tank 485 isconnected to the single unit 400 a.

[0298] The above arrangement where the reservoir tank 485 issupportingly connected to the single unit 400 a can omit externalconduits for feeding the hydraulic fluid from the reservoir tank 485 tothe charge pump 450, and external conduits for returning the hydraulicfluid from the single unit 400 a to the reservoir tank 485, therebyachieving ease of assembly, lower manufacturing cost, improvedefficiency in replenishing the hydraulic fluid through the decrease ofthe resistance force between the hydraulic fluid and the conduit wall,and producing other desirable effects.

[0299] The reservoir tank 485 preferably communicates with the housing420 via a hydraulic fluid communication passage 487 such as a pipe, asillustrated in FIGS. 34 and 35. This hydraulic communication allows bothreservoir tank 485 and housing 420 to be used as a hydraulic fluid tank,and hence the reservoir tank itself to have a reduced size. In thisarrangement, the reservoir tank is preferably located so that the upperlevel of the hydraulic fluid within the reservoir tank can be higherthan the upper end of the housing. This arrangement produces anadditional desirable effect, and more specifically allows the completefilling of the hydraulic fluid within the housing 420, therebyeffectively avoiding the air entrained in the hydraulic fluid. Inaddition, variation in volume of the hydraulic fluid within the housing420 due to variation in temperature of this hydraulic fluid can beproperly compensated by the reservoir tank 485 communicating with thehousing.

[0300] The reservoir tank 485 can be connected to the inlet port forcharging 434 via a hydraulic fluid replenishing passage 488 such as apipe. According to the pump unit of this embodiment, which includes thecharge pump 450 serving as the hydraulic fluid feeding means, asdescribed above, the reservoir tank 485 communicates, via the hydraulicfluid replenishing passage 488, with the inlet port 456 communicatingwith the inlet port 455 of the charge pump 450, and the outlet port 451of the charge pump 450 communicates with the inlet port for charging 434via the cartridge filter 486 (see FIGS. 34 and 37).

[0301] The communications between the reservoir tank 485 and the inletport 434 via the hydraulic fluid replenishing passage 488, and betweenthe reservoir tank 485 and the housing 420 via the hydraulic fluidcommunication passage 487 can reduce the number of the conduits requiredrespectively between the first and second hydraulic pumps, and the firstand second hydraulic motors to substantially four conduits only,specifically the first pair of hydraulic lines 484 a and the second pairof hydraulic lines 484 b. Thus, as compared with the conventionalarrangements as disclosed in the above cited U.S. Pat. No. 4,920,733,the pump unit of this embodiment can achieve a lower manufacturing cost,an improved assembling efficiency and an excellent workability inmaintenance.

[0302] The pump unit 400 more preferably includes a cooling fan 481disposed near the single unit 400 a and the reservoir tank 485 andoperatively driven by the power source 480. According to thisarrangement with the cooling fan 481, the reservoir tank 485 isconnected to the single unit 400 a in such a manner as to form betweenthe reservoir tank 485 and the single unit 400 a a clearance 489 intowhich a cooling air stream is drawn from the cooling fan 481. Thehydraulic fluid replenishing passage 488 and/or the hydraulic fluidcommunication passage 487 traverses the clearance 489. In thisarrangement, the hydraulic fluid replenishing passage 488 and thehydraulic fluid communication passage 487 each may have the right andleft sides surrounded by a cooling air duct or shroud to effectivelyguide the cooling air stream from the cooling fan to the clearance 489.

[0303] The thus arranged pump unit 400 can limit the increase intemperature of the hydraulic fluid stored in the reservoir tank 485 andthe housing 420, and also effectively limit the increase in temperatureof the hydraulic fluid flowing through the hydraulic fluid replenishingpassage 488 and the hydraulic fluid communication passage 487. Thus, thetransmission efficiency between the hydraulic pumps and the hydraulicmotors can be improved.

[0304] The hydraulic fluid replenishing passage and the hydraulic fluidcommunication passage each more preferably has an outer circumferenceprovided with fins (not shown) to obtain an enlarged heat radiatingarea, and hence an improved cooling efficiency. The fins can also beprovided on the reservoir tank 485 itself.

[0305] Preferably, the reservoir tank 485 is made of a semitransparentresin material to afford a visual observation of the level of thehydraulic fluid from the outside of the tank. The reservoir tank 485 canalso include a tank cap 485 b with an air release mechanism on the topof the tank.

[0306] In this embodiment, the charge pump 450 is provided to forciblyfeed the pressurized hydraulic fluid to the inlet port for charging 434.Alternative to this arrangement with the charge pump 450, the inlet portfor charging 434 may be directly connected to the reservoir tank 485 viathe hydraulic fluid replenishing passage 488, thereby allowing thehydraulic fluid to spontaneously flow into the inlet port 434 when thepressure in a lower pressure line of the first pair of hydraulic lines484 a and/or the pressure in a lower pressure line of the second pair ofhydraulic lines 484 b drops from a predetermined value.

Eighth Embodiment

[0307] Another embodiment of the pump unit according to the fourthaspect of the present invention will be hereinafter described withreference to FIGS. 41 and 42. FIG. 41 is a longitudinal cross-sectionalside view of the pump unit 400′, and FIG. 42 is a cross section takenalong lines XXXXII-XXXXII in FIG; 41.

[0308] As illustrated in FIG. 41, the pump unit 400′ of this embodimentis a tandem pump unit with the first hydraulic pump 410 a connected inseries with the second hydraulic pump 410 b. In the followingdescription, corresponding or identical parts to those of the seventhembodiment have been given the same reference characters or those withprimes (′) to omit a detailed description thereof.

[0309] As illustrated in FIG. 41, the pump unit 400′ includes the commonhousing 420′ for accommodating the first hydraulic pump 410 a and thesecond hydraulic pump 410 b, and the first center section 430 a and thesecond center section 430 b respectively supporting the first hydraulicpump 410 a and the second hydraulic pump 410 b.

[0310] The common housing 420′ has the first end (the lower end in thisembodiment), and the second end (the upper end in this embodiment) alongthe axial direction thereof respectively defining the first opening 420a′ for receiving the first hydraulic pump 410 a and the second opening420 b′ for receiving the second hydraulic pump 410 b.

[0311] The common housing 420′ also forms the partition wall 420 c′ atsubstantially the center in the direction of the pump shaft to dividethe common housing into the first pump accommodation chamber and thesecond pump accommodation chamber. The partition wall 420 c′ includes abearing portion for supporting the connection portion between the firstpump shaft 411 a and the second pump shaft 411 b. Specifically, thepartition wall 420 c′ includes a connection member 416 non-rotatablyfixed around the downstream end or the upper end of the first pump shaft411 a and the upstream end or the lower end of the second pump shaft 411b, and rotatably supported in the bearing hole 420 d′ formed in thepartition wall. The partition wall 420 c′ may form a plurality ofhydraulic fluid communication passages 420 e′ for communication betweenthe first pump accommodation chamber and the second pump accommodationchamber. These communication passages enable the entire housing to beused as the hydraulic fluid tank.

[0312] The first center section 430 a supports on the upper surfacethereof the first hydraulic pump 410 a, and is connected to the housing420′ in such a manner as to seal the first opening 420 a′ of thehousing. The first pump shaft 411 a of the first hydraulic pump 410 ahas the upstream end or the lower end extending downwardly through thefirst center section 430 a to form a lower extension through which thepower is inputted to drive the hydraulic pump units and the cooling fan481.

[0313] On the other hand, the second center section 430 b supports onthe lower surface thereof the second hydraulic pump 410 b, and isconnected to the housing 420′ in such a manner as to seal the secondopening 420 b′ of the housing 420′. The second pump shaft 411 b of thesecond hydraulic pump 410 b has the downstream end or the upper endextending upwardly through the second center section 430 b to form anupper extension through which the charge pump 450 is driven.

[0314] The first center section 430 a, as illustrated in FIGS. 33 and41, forms a first pair of hydraulic passages 431 a for the firsthydraulic pump, respectively having first ends opening to the outside ofthe first center section through the surface facing the first pistonunit 412 a (the upper surface) to respectively communicate with theinlet/outlet ports of the first piston unit, and second ends opening tothe outside of the first center section. The second ends of the firstpair of hydraulic passages 431 a opening to the outside forms a firstpair of inlet/outlet ports 432 a respectively serving as connectionports for connection with the first pair of hydraulic lines 484 aextending to the first hydraulic motor 482 a.

[0315] Similarly, the second center section 430 b, as illustrated inFIGS. 33, 41 and 41, forms a second pair of hydraulic passages 431 b forthe second hydraulic pump, respectively having first ends opening to theoutside of the second center section through the surface facing thesecond piston unit 412 b to respectively communicate with theinlet/outlet ports of the second piston unit, and second ends opening tothe outside of the second center section. The second ends of the secondpair of hydraulic passages 431 b opening to the outside forms a secondpair of inlet/outlet ports 432 b respectively serving as connectionports for connection with the second pair of hydraulic lines 484 bextending to the second hydraulic motor 482 b.

[0316] Similarly to the seventh embodiment, the pump unit 400′ of thisembodiment includes the common charging passage 433 disposed therein,having a first end opening to the outside of the pump unit to form theinlet port for charging 434, and the second end communicating with thefirst and second pairs of hydraulic passages.

[0317] The common charging passage 433, as illustrated in FIGS. 41 and42, includes a first bore portion 433 a, a conduit portion 433 b and asecond bore portion 433 c. The first bore portion 433 a is formed in thesecond center section 430 b to have a first end opening to the outsideof the second center section through the upper surface thereof to formthe inlet port for charging 434 and a second end communicating with thesecond pair of hydraulic passages 431 b via the check valves 461 c and461 d and opening to the second pump accommodation chamber. The conduitportion 433 b is disposed to have a first end connected to the secondend of the first bore portion 433 a and a second end extending throughthe second pump accommodation chamber, the partition wall 420 c and thefirst pump accommodation chamber to the first center section 430 a. Thesecond bore portion 433 c is formed in the first center section 430 a tohave a first end connected to the second end of the conduit portion 433b and a second end communicating with the first pair of hydraulicpassages 431 a via the check valves 461 a and 461 b. The conduit portion433 b can be extended through the partition wall 420 c′ by disposing theconduit portion 433 b within one of the plurality of hydraulic fluidcommunication passages 420 e′.

[0318] Connected to the common charging passage 433 is a pressure reliefline 453 communicating with the housing via a relief valve 452. Thepressure relief line 453, similarly to the seventh embodiment, is formedin the charge pump casing 459 to communicate with the housing 420′ viathe drain port 435 formed in the second center section 430 b.

[0319] The thus arranged pump unit 400′ of this embodiment also producesthe same effects as those of the seventh embodiment.

[0320] Alternative to the conduit portion 433 b′, it is possible to formin the peripheral wall of the common housing 420 a communication holehaving a first end connected to the second end of the first bore portion433 a′ and a second end connected to the first end of the second boreportion 433 c′.

Ninth Embodiment

[0321] One embodiment of the pump unit according to the fifth aspect ofthe present invention will be hereinafter described with reference tothe accompanying drawings. FIG. 43 is a hydraulic circuit diagram of thevehicle to which a pump unit 500 of this embodiment is applied. FIG. 44is a longitudinal cross-sectional side view of the pump unit and itsperiphery. FIGS. 45 to 48 are respectively cross sections taken alonglines XXXXV-XXXXV, XXXXVI-XXXXVI, XXXXVII-XXXXVII, andXXXXVIII-XXXXVIII.

[0322] As illustrated in FIGS. 43 to 45, the pump unit 500 is adapted tobe used in a vehicle having right and left drive wheels 583 a and 583 bto which first and second hydraulic motors 582 a and 582 b arerespectively connected, and includes a first hydraulic pump 510 a and asecond hydraulic pump 510 b respectively connected to the first andsecond hydraulic motors 582 a and 582 b via a first pair of hydrauliclines 584 a and a second pair of hydraulic lines 584 b, and a commonhousing 520 for accommodating these hydraulic pumps 510 a and 510 b.

[0323] The connection form between the right and left drive wheels 583 aand 583 b, and the first and second hydraulic motors 582 a and 582 bmeant in this embodiment includes the direct connection of the drivewheels respectively to those hydraulic motors, and also an operativeconnection of the drive wheels respectively to the hydraulic motors viaa suitable power transmission mechanism. In FIG. 43, the reference codes580, 581 and 586 respectively represent a power source, a cooling fanand a filter.

[0324] As illustrated in FIGS. 44 to 45, the first hydraulic pump 510 aand the second hydraulic pump 510 b are axial piston pumps of a variabledisplacement type, and respectively include a first pump shaft 511 a anda second pump shaft 511 b that have vertical axes and are disposedparallel to one another in the vehicle width direction within thehousing 520, a first piston unit 512 a and a second piston unit 512 bthat are reciprocatingly movable according to the rotation of the pumpshafts, a first cylinder block 513 a and a second cylinder block 513 bthat reciprocably support the piston units, a first angularly adjustableswash plate 514 a and a second angularly adjustable swash plate 514 bthat regulate the stroke length of the piston units by varying theirtilting angles to vary the input/output flow rates of the piston units,and a first control shaft 515 a and a second control shaft 515 b thatcontrol the tilting angles of these swash plates.

[0325] The pump unit of this embodiment is of a vertical type with thefirst and second pump shafts 511 a and 511 b having the vertical axes.However, the second aspect of the present invention is not necessarilylimited to this arrangement. It is a matter of course that the pump unit500 can be of a horizontal type with the first and second pump shafts511 a and 511 b having the horizontal axes.

[0326] As best illustrated in FIG. 45, the first and second angularlyadjustable swash plates 514 a and 514 b of this embodiment are of acradle type.

[0327] As illustrated in FIGS. 44 and 48, the first control shaft 515 aand the second control shaft 515 b respectively have inner endsextending into the housing 520 to be respectively connected to arms 516a and 516 b and hence the first and second swash plates 514 a and 514 b,and outer ends extending rearwards in the vehicle longitudinaldirection.

[0328] Alternative to the above arrangement, the first and secondcontrol shafts 515 a and 515 b may extend away from one another in thevehicle width direction to respectively have oppositely positioned outerends. This arrangement is advantageous when installed on the vehiclehaving push-pull control levers 198 a and 198 b as illustrated in FIG.1, since the first and second control shafts 515 a and 515 b can havethe rotating shaft centers disposed parallel to the longitudinal axis ofthe control levers, thereby achieving the simplification of a linkmechanism between these control shafts and the control levers.

[0329] In the above arrangement, the first control shaft 515 a and thesecond control shaft 515 b are more preferably located at substantiallythe same position with respect to the vehicle longitudinal direction.The thus arranged first and second control shafts 515 a and 515 b can bealigned with the control levers in the vehicle width direction, therebyachieving a more simplified structure of the link mechanism.

[0330] The pump unit 500 further includes a common center section 530that supports the first and second hydraulic pumps 510 a and 510 b, anda power transmission mechanism 540 that is accommodated within thehousing 520 to operatively connect the first and second hydraulic pumpshafts 511 a and 511 b together.

[0331] The pump unit 500 with the power transmission mechanism 540permits the simultaneous rotation of both pump shafts 511 a and 511 bonly by connecting the power source to either one of the first andsecond pump shafts 511 a and 511 b, or to the first pump shaft 511 a inthis embodiment, thereby achieving the simplified structure for thepower transmission from the power source to the pump unit 500. In thisembodiment, the power transmission mechanism 540 is in the form of agear transmission device that includes a first gear 540 a non-rotatablysupported on the lower side of the first pump shaft 511 a, and a secondgear 540 b non-rotatably supported on the lower side of the second pumpshaft 511 b in meshed engagement with the first gear 540 a (see FIGS. 45and 46). Instead of the gear transmission device, any conventional powertransmission mechanisms such as chain and belt may be used.

[0332] The housing 520, as illustrated in FIGS. 44 and 45, includes afirst housing 521 for accommodating the first and second hydraulic pumps510 a and 510 b, and a second housing 522 for accommodating the powertransmission mechanism 540.

[0333] The first housing 521 has a box shape with a first sidewall 521 adisposed in the upper or lower side of the pump shafts 511 a and 511 balong the longitudinal direction thereof, or in this embodiment in thelower side of the pump shafts 511 a and 511 b, which will be hereinafterreferred to simply as the lower side, and a peripheral wall 521 bextending from a peripheral edge of the first sidewall 521 a to theopposite side of the pump shafts 511 a and 511 b along the longitudinaldirection thereof (i.e., the upper side of the pump shafts 511 a and 511b in this embodiment, which will be referred to simply as the upperside). The first sidewall 521 a forms bearing holes through which thefirst and second pump shafts 511 a and 511 b respectively extend. Theupper side of the first housing 521 has an end surface forming anopening through which the first and second hydraulic pumps 510 a and 510b can be placed into the first housing 521. The opening of the firsthousing 521 is sealed by the center section 530 in a liquid tightmanner. That is, the center section 530 of this embodiment constitutes apart of the first housing 521.

[0334] The second housing 522 is disposed in the lower side, and has abox shape with a lower sidewall 522 a forming a bearing hole throughwhich the lower end of the first pump shaft 511 a extends and a bearingportion for receiving the lower end of the second pump shaft 511 b, anda peripheral wall 522 b extending upwardly from a peripheral edge of thelower sidewall 522 a. The upper side of the second housing 522 forms anopening through which the power transmission mechanism 540 can be placedinto the second housing 522.

[0335] The second housing 522 is connected to the first housing 521 insuch a manner as to have the opening sealed in a liquid tight manner bythe first sidewall 521 a of the first housing 521, and form anaccommodation space of the power transmission mechanism 540 incooperation with the first sidewall 521 a of the first housing 521.

[0336] In the thus arranged housing 520, the first sidewall 521 a of thefirst housing 521 serves as a partition wall dividing the accommodationspace of the housing into a hydraulic pump accommodation chamber and apower transmission mechanism accommodation chamber. The partition wallthus defining the hydraulic pump accommodation chamber and the powertransmission mechanism accommodation chamber can effectively prevent anyforeign matters such as iron powder generated in the power transmissionmechanism 540 from intruding into the hydraulic pump accommodationchamber, and hence damaging piston units 512 a, 512 b, cylinder blocks513 a, 513 b, and/or other parts. In addition to this foreign matterprevention measure, the first and second pump shafts 511 a and 511 b,which extend through the partitio n wall 521 a, may have circumferentialperipheries with seal rings thereon to more securely prevent theintrusion of the foreign matters.

[0337] Portions of the housing 520, through which the respective shafts511 a, 515 a and 515 b extend, are sealed by any suitable sealing meansin a liquid tight manner, thereby allowing the housing 520 to serve asthe hydraulic fluid tank.

[0338] The first sidewall 521 a serving as the partition wall preferablyforms a hydraulic fluid communication hole (not shown) for communicationbetween the hydraulic pump accommodation chamber and the powertransmission mechanism accommodation chamber, with a filter provided inthe hole for preventing the intrusion of the foreign matters into thehydraulic pump accommodation chamber. The thus formed hydraulic fluidcommunication hole can omit the necessity of separately feeding thelubricant to the power transmission mechanism 540, with the result thatthe power transmission mechanism 540 can be lubricated with thehydraulic fluid stored within the housing. This permits lowmanufacturing cost and ease of maintenance.

[0339] In this embodiment, the first and second angularly adjustableswash plates 514 a and 514 b are of a cradle type, as illustrated inFIG. 45. Therefore, when the partition wall 521 a forms, on its sidefacing the hydraulic pumps 510 a, 510 b, spherical concave surfaces 524respectively adapted to spherical convex surfaces 519 formed in the rearsides of the swash plates 514 a and 514 b, which rear sides beingopposite to the surfaces facing the piston units 512 a and 512 b, thespherical concave surfaces 524 can slidingly guide the spherical convexsurfaces 519 of the swash plates 514 a and 514 b. The swash plates thuscan securely rest on the spherical concave surfaces 524.

[0340] In this embodiment, the first sidewall 521 a of the first housing521 serves as the partition wall. Alternatively, a partitioning meansmay take various forms, as long as it can produce the same effect asdescribed above. For example, a separately prepared partition wall maybe mounted in a housing having a simple cylindrical box shape (see FIG.14).

[0341] Now, the description will be made for the center section 530.FIG. 49 is a cross section taken along lines XXXXIX-XXXXIX in FIG. 47.As best illustrated in FIGS. 47 and 49, the center section 530 forms afirst pair of hydraulic passages 531 a respectively having first endscommunicating with the first piston unit and second ends opening to theoutside of the center section 530 to form a first pair of inlet/outletports 532 a serving as connection ports for connection with the firstpair of hydraulic lines 584 a (see FIG. 43).

[0342] The center section 530, as best illustrated in FIG. 47, alsoforms a second pair of hydraulic passages 531 b having first endscommunicating with the second piston unit and second ends opening to theoutside of the center section 530 to form a second pair of inlet/outletports 532 b serving as connection ports for connection with the secondpair of hydraulic lines 584 b (see FIG. 43).

[0343] The common center section 530 thus forms all the first and secondpairs of inlet/outlet ports 532 a and 532 b serving as the connectionports for connection with the first and second pairs of hydraulic lines584 a and 584 b. Whereby, the piping work between the hydraulic pumps510 a and 510 b, and the hydraulic motors 582 a and 582 b can befacilitated. In this embodiment, the common center section 530 has sidesurfaces facing one another that respectively form the first pair ofinlet/outlet ports 532 a and the second pair of inlet/outlet ports 532b. Alternatively, the first and second pairs of inlet/outlet ports 532 aand 532 b can be formed in the same side of the center section, therebyfurther facilitating the piping work.

[0344] The center section 530, as illustrated in FIGS. 44, 46 and 47,also forms a common charging passage 533 having a first end opening tothe outside of the center section 530 to form an inlet port for charging534 serving as an inlet for the hydraulic fluid to be replenished, and asecond end communicating with the first pair of hydraulic passages 531 aand the second pair of hydraulic passages 531 b via check valves 561 a,561 b, 561 c and 561 d.

[0345] In this embodiment, the first pump shaft 511 a, as bestillustrated in FIGS. 44 and 45, has an extension extending upwards fromthe upper end thereof to be located above the center section 530,thereby supporting a charge pump 550 via the extension. The charge pump550 has an upper surface with a cartridge filter 586 detachably mountedthereto, through which the hydraulic fluid is sucked into the inlet port555 of the charge pump 550. The cartridge filter 586 can be provided atthe discharge side of the charge pump 550.

[0346] The charging passage 533 is connected to a first end of apressure relief line 553 with a relief valve 552 therein. The reliefvalve 552 regulates the hydraulic pressure of the charging passage 533(see FIGS. 43 and 44).

[0347] The pressure relief line 533 has a second end opening to theoutside to form a drain port 554 through which the hydraulic fluid fromthe relief valve is drained.

[0348] In this embodiment, the pressure relief line 553 is formed in acharge pump casing 559 mounted on the upper surface of the centersection 530. However, the present invention is not necessarily limitedto this arrangement. That is, the pressure relief line 553 can be formedin the center section 530.

[0349] Reference codes 551 and 556 in FIGS. 44 and 46 respectivelyrepresent an outlet port of the charge pump, and an inlet portcommunicating with the inlet port 555 of the charge pump 550 via thefilter 586.

[0350] As described above, the charging passage 533 has the second endcommunicating with the first pair of hydraulic passages 531 a and thesecond pair of hydraulic passages 531 b via the check valves 561 a, 561b, 561 c and 561 d so as to allow the pressurized hydraulic fluid to befed from the common charging passage 533 into a lower pressure line ofthe first pair of hydraulic lines 584 a and a lower pressure line of thesecond pairs of hydraulic lines 584 b, while preventing the pressurizedhydraulic fluid from flowing in the reverse direction.

[0351] Further, bypass lines 562 a and 562 b having throttle valves areformed between at least one of the first pair of hydraulic passages 531a and the charging passage 533, and between at least one of the secondpair of hydraulic passages 531 b and the charging passage 533 (see FIGS.43 and 47).

[0352] The bypass lines 562 a and 562 b are designed to assure theneutralization of the hydraulic pumps 510 a and 510 b. Specifically,even if the swash plates 514 a and/or 514 b of the hydraulic pumps 510 aand 510 b tilts from the neutral positions by a small angle, thereoccurs the pressure difference between the first pair of hydraulic lines484 a, and/or between the second pair of hydraulic lines 454 b. Thispressure difference causes the rotation of the hydraulic motors 582 aand 582 b. That is, even a slight amount of the displacement between theactual neutral positions and the predetermined design positions of theswash plates 514 a and 514 b due to assembling errors or the like causesan unintentional rotation of the hydraulic motors 582 a and 582 b. Onthe contrary, the bypass lines 562 a and 562 b, as described above,allow the pressurized hydraulic fluid to leak therethrough from thefirst pair of hydraulic lines 584 a and the second pair of hydrauliclines 584 b. Thus, the pressure difference between the pair of firsthydraulic lines 584 a and/or between the second pair of hydraulic lines584 b can effectively be limited, thereby effectively avoiding theunintentional rotation of the hydraulic motors 582 a and 582 b, even forthe swash plates 514 a and 514 b having the actual neutral positiondisplaced from the design neutral position due to the assembling errorsor the like.

[0353] In view of transmission efficiency between the hydraulic pumps510 a, 510 b and the hydraulic motors 582 a, 582 b, the leakage of thepressurized hydraulic fluid from the first and second pairs of hydrauliclines 584 a, 584 b through the bypass lines 562 a, 562 b is notpreferable. Therefore, the bypass lines 562 a, 562 b are preferablyprovided in portions from the charging passage 533 to one of the firstpair of hydraulic passages 531 a, and to one of the second pair ofhydraulic passages 531 b.

[0354] The check valves 561 a, 561 b, 561 c and 561 d are morepreferably provided with release means 562 to forcibly bring the firstpair of hydraulic passages 531 a into communication with one another,and the second pair of hydraulic passages 531 b into communication withone another, if an emergency arises, as illustrated in FIG. 47. Therelease means 562 are designed to easily move the vehicle, when thevehicle must forcibly be moved or the vehicle wheels must forcibly berotated by man power or the like due to the disorder of the power source580, the hydraulic pumps 510 a, 510 b or the like. Specifically, whenthe vehicle wheels connected to the hydraulic motors 582 a and 582 b areforcibly rotated with the first pair of hydraulic lines 584 a and/or thesecond pair of hydraulic lines 584 b lying in the closing state, thereoccurs the pressure difference between the first pair of hydraulic lines584 a, and between the second pair of hydraulic lines 584 b. As aresult, the vehicle is hardly moved, or the vehicle wheels are hardlyrotated. On the contrary, the release means can easily achieve thecommunications between the first pair of hydraulic passages 531 a, andbetween the second pair of hydraulic passages 531 b by mechanicallyreleasing all the check valves 561 a to 561 d. Whereby, the vehicle caneasily be moved by man power or the like.

[0355] As illustrated in FIG. 47, all the release means 563 arepreferably disposed in the same side of the center section 530, so thatthe link mechanism linking these release means 563 for operation of thesame can have a simplified structure.

[0356] As described above, the pump unit 500 of this embodiment includesthe first and second hydraulic pumps 510 a and 510 b, the center section530 and the housing 520, all of which are integrally connected togetherto constitute a single unit 500 a. Accordingly, both first and secondhydraulic pumps 510 a and 510 b can be installed on the vehicle only bymounting the single unit 500 a on the vehicle, thereby achieving animproved efficiency in assembling the vehicle.

[0357] The pump unit 500 of this embodiment also includes a reservoirtank 585 supportingly connected to the single unit 500 a, as illustratedin FIGS. 44 to 46. In this embodiment, the reservoir tank 585 has rightand left sides respectively forming mounting ribs 585 a, so that thereservoir tank 585 is supportingly connected to the single unit 500 avia mounting members 590 fastened to the mounting ribs 585 a.

[0358] The reservoir tank 585 communicates with the housing 520 for afree fluid communication therebetween via a hydraulic fluid replenishingpassage 587 or other suitable conduits means, as illustrated in FIGS. 44and 48. This hydraulic communication allows the reservoir tank 585 to beused as a hydraulic fluid tank together with the housing 520.

[0359] Preferably, the reservoir tank 585 has the upper side positionedhigher than the upper side of the housing 520, so that the housing canbe completely filled with the hydraulic fluid, thereby effectivelypreventing the air from being entrained in the hydraulic fluid storedwithin the housing 520. Variation in volume of the hydraulic fluidwithin the housing 520 due to variation in temperature of this hydraulicfluid can be properly compensated by the reservoir tank 585communicating with the housing.

[0360] The hydraulic fluid tank communicates with the inlet port forcharging 534. According to this embodiment, the hydraulic fluid, whichhas been sucked via the hydraulic fluid replenishing passage 588 fromthe reservoir tank 585 constituting a part of the hydraulic tank, is fedinto the inlet port for charging 534 via the pressure relief line 553(see FIGS. 43 and 44).

[0361] On the other hand, the drain port adapted to drain the hydraulicfluid from the relief valve 552 installed within the pressure reliefline 553 is connected to a cooling conduit 591 via a first end thereof.The cooling conduit 591 has a second end communicating with thereservoir tank 585 constituting a part of the hydraulic tank. Thecooling conduit 591, as illustrated in FIG. 44, has at least a portionextending through the outside air with a spacing from the single unit500 a and the reservoir tank 585 to air-cool the hydraulic fluid flowingthrough the cooling conduit 591. The cooling conduit 591 preferably hasan outer circumference provided with cooling fins to obtain an enlargedheat radiation area, and hence improved cooling efficiency.

[0362] The cooling conduit 591 may be connected to the reservoir tank585 or the single unit 500 a by a suitable bridging means.

[0363] The pump unit is thus designed so that the hydraulic fluid, whichhas been sucked into the inlet port 555 of the charge pump 550 via thehydraulic fluid replenishing passage 588, and discharged through theoutlet port 551, partly returns to the hydraulic fluid tank via thecooling conduit 591 extending through the outside air.

[0364] Specifically, the hydraulic fluid replenishing passage 588, apart of the pressure relief line 553 and the cooling conduit 591together constitutes a circulation line having a first end communicatingwith the hydraulic fluid tank and a second end again communicating withthe hydraulic fluid tank. The charge pump 550 is also designed to allowthe hydraulic fluid to be sucked through the first end of thecirculation line and to be returned to the hydraulic fluid tank throughthe second end of the circulation line. Whereby, the rise in temperatureof the stored hydraulic fluid can effectively be prevented. As a result,deterioration in working efficiency of the hydraulic pumps and thehydraulic motors can effectively be prevented.

[0365] The pump unit of this embodiment has the arrangement to allow thehydraulic fluid drained from the relief valve 552 installed within thepressure relief line 553 to be returned to the hydraulic fluid tank viathe cooling conduit 591 in consideration of the cooling efficiency ofthe hydraulic fluid. That is, the hydraulic fluid discharged from thecharge pump 550 is highly pressurized, and therefore has a hightemperature due to the pressure energy of the discharged hydraulicfluid. Therefore, when the drained hydraulic fluid is directly returnedto the hydraulic fluid tank, the temperature of the hydraulic fluidstored in the hydraulic tank may increase. On the contrary, the pumpunit of this embodiment includes the cooling conduit 591 to return thehydraulic fluid of a high temperature drained from the relief valve 552to the hydraulic fluid tank to effectively limit the increase intemperature of the hydraulic fluid stored within the tank.

[0366] More preferably, the cooling fan 581 operatively driven by thepower source 580 is disposed near the single unit 500 a and thereservoir tank 585, and the reservoir tank 585 is connected to thesingle unit 500 a in such a manner as to form between the reservoir tank585 and the single unit 500 a a clearance 589 into which a cooling airstream is drawn from the cooling fan 581. The hydraulic fluidreplenishing passage 588 and/or the hydraulic fluid communicationpassage 587 traverses the clearance 589.

[0367] In the above arrangement, the hydraulic fluid replenishingpassage 588 and the hydraulic fluid communication passage 587 eachpreferably have the right and left sides surrounded by a cooling airduct or shroud to effectively guide the cooling air stream from thecooling fan to the clearance 589.

[0368] In this embodiment, the mounting members 590 for mounting thereservoir tank 585 to the single unit 500 a is formed into a casing (seeFIG. 50) with sidewalls, so that the cooling air stream from the coolingfan 581 can efficiently drawn into the clearance along its sidewalls.

[0369] Such an additional fluid cooling arrangement can achieve coolingof the hydraulic fluid flowing through the hydraulic fluid replenishingpassage 588 and the hydraulic fluid communication passage 587 inaddition to the cooling of the hydraulic fluid flowing through thecooling conduit 591, thereby more effectively limiting the increase intemperature of the hydraulic fluid within the hydraulic tank.

[0370] The hydraulic fluid replenishing passage and the hydraulic fluidcommunication passage each more preferably has an outer circumferenceprovided with cooling fins (not shown) to obtain an enlarged heatradiating area, and hence an improved cooling efficiency. The coolingfins can also be provided on the reservoir tank 585 itself.

[0371] Preferably, the reservoir tank 585 is made of a semitransparentresin material to afford a visual observation of the level of thehydraulic fluid from the outside of the tank. The reservoir tank 585 canalso include a tank cap 585 b with an air release mechanism on the topof the tank.

[0372] In this embodiment, the charge pump 550 is provided to forciblyfeed the pressurized hydraulic fluid to the inlet port for charging 534,and to circulate the hydraulic fluid through the circulation line. Thepresent invention is not necessarily limited to this arrangement.Alternative to this arrangement with the charge pump 550, the inlet portfor charging 534 may be directly connected to the reservoir tank 585 viathe hydraulic fluid replenishing passage 588 by omitting the charge pump550. This allows the hydraulic fluid to spontaneously flow into theinlet port 534 when the pressure in a lower pressure line of the firstpair of hydraulic lines 584 a and/or the pressure in a lower pressureline of the second pair of hydraulic lines 584 b drops from apredetermined value. In addition, a pump may be separately provided tocirculate the hydraulic fluid through the circulation line.

[0373] The pump unit of this embodiment may employ the arrangement,which allows a cooling air stream from a cooling fan (not shown) for thepower source or a radiator (not shown) to be applied on the coolingconduit 591. This arrangement can more effectively cool the hydraulicfluid flowing through the cooling conduit 591.

Tenth Embodiment

[0374] Another embodiment of the pump unit according to the fifth aspectof the present invention will be hereinafter described with reference toFIG. 51. FIG. 51 is a longitudinal cross-sectional side view of the pumpunit 500′.

[0375] As illustrated in FIG. 51, the pump unit 500′ of this embodimentis a tandem pump unit with the first hydraulic pump 510 a connected inseries with the second hydraulic pump 510 b. In the followingdescription, corresponding or identical parts to those of the ninthembodiment have been given the same reference characters or those withprimes (′) to omit a detailed description thereof.

[0376] As illustrated in FIG. 51, the pump unit 500′ includes the commonhousing 520′ for accommodating the first hydraulic pump 510 a and thesecond hydraulic pump 510 b, and the first center section 530 a and thesecond center section 530 b respectively supporting the first hydraulicpump 510 a and the second hydraulic pump 510 b.

[0377] The common housing 520′ has the first end (the lower end in thisembodiment), and the second end (the upper end in this embodiment) alongthe axial direction thereof respectively defining the first opening 520a′ for receiving the first hydraulic pump 510 a and the second opening520 b′ for receiving the second hydraulic pump 510 b.

[0378] The common housing 520′ also forms the partition wall 520 c′ atsubstantially the center in the direction of the pump shaft to dividethe common housing into the first pump accommodation chamber and thesecond pump accommodation chamber. The partition wall 520 c′ includes abearing portion for supporting the connection portion between the firstpump shaft 511 a and the second pump shaft 511 b. Specifically, thepartition wall 520 c′ includes a connection member 516 non-rotatablyfixed around the downstream end or the upper end of the first pump shaft511 a and the upstream end or the lower end of the second pump shaft 511b, and rotatably supported in the bearing hole 520 d′ formed in thepartition wall. The partition wall 520 c′ may form a plurality ofhydraulic fluid communication passages 520 e′ for communication betweenthe first pump accommodation chamber and the second pump accommodationchamber. These communication passages enable the entire housing to beused as the hydraulic fluid tank.

[0379] The first center section 530 a supports on the upper surfacethereof the first hydraulic pump 510 a, and is connected to the housing520′ in such a manner as to seal the first opening 520 a′ of thehousing. The first pump shaft 511 a of the first hydraulic pump 510 ahas the upstream end or the lower end extending downwardly through thefirst center section 530 a to form a lower extension through which thepower is inputted to drive the hydraulic pump units and the cooling fan581.

[0380] On the other hand, the second center section 530 b supports onthe lower surface thereof the second hydraulic pump 510 b, and isconnected to the housing 520′ in such a manner as to seal the secondopening 520 b ′ of the housing 520′. The second pump shaft 511 b of thesecond hydraulic pump 510 b has the downstream end or the upper endextending upwardly through the second center section 530 b to form anupper extension through which the charge pump 550 is driven.

[0381] The first center section 530 a, as illustrated in FIGS. 43 and51, forms a first pair of hydraulic passages 531 a for the firsthydraulic pump, respectively having first ends opening to the outside ofthe first center section through the surface facing the first pistonunit 512 a (the upper surface) to respectively communicate with theinlet/outlet ports of the first piston unit, and second ends opening tothe outside of the first center section. The second ends of the firstpair of hydraulic passages 531 a opening to the outside forms a firstpair of inlet/outlet ports 532 a respectively serving as connectionports for connection with the first pair of hydraulic lines 584 aextending to the first hydraulic motor 582 a.

[0382] Similarly, the second center section 530 b, as illustrated inFIGS. 43 and 51, forms a second pair of hydraulic passages 531 b for thesecond hydraulic pump, respectively having first ends opening to theoutside of the second center section through the surface facing thesecond piston unit 512 b to respectively communicate with theinlet/outlet ports of the second piston unit, and second ends opening tothe outside of the second center section. The second ends of the secondpair of hydraulic passages 531 b opening to the outside forms a secondpair of inlet/outlet ports 532 b respectively serving as connectionports for connection with the second pair of hydraulic lines 584 bextending to the second hydraulic motor 582 b.

[0383] Similarly to the ninth embodiment, the pump unit 500′ of thisembodiment includes the common charging passage 533 disposed therein,having a first end opening to the outside of the pump unit to form theinlet port for charging 534, and the second end communicating with thefirst and second pairs of hydraulic passages.

[0384] The common charging passage 533, as illustrated in FIGS. 51,includes a first bore portion 533 a, a conduit portion 533 b and asecond bore portion 533 c. The first bore portion 533 a is formed in thesecond center section 530 b to have a first end opening to the outsideof the second center section through the upper surface thereof to formthe inlet port for charging 534 and a second end communicating with thesecond pair of hydraulic passages 531 b via the check valves 561 c and561 d and opening to the second pump accommodation chamber. The conduitportion 533 b is disposed to have a first end connected to the secondend of the first bore portion 533 a and a second end extending throughthe second pump accommodation chamber, the partition wall 520 c and thefirst pump accommodation chamber to the first center section 530 a. Thesecond bore portion 533 c is formed in the first center section 530 a tohave a first end connected to the second end of the conduit portion 533b and a second end communicating with the first pair of hydraulicpassages 531 a via the check valves 561 a and 561 b. The conduit portion533 b can be extended through the partition wall 520 c′ by disposing theconduit portion 533 b within one of the plurality of hydraulic fluidcommunication passages 520 e′.

[0385] The charging passage 533 is also connected to the pressure reliefline 553 via the first end thereof with the relief valve 552 installedtherein in the same manner as the ninth embodiment. The relief valve 552regulates the hydraulic pressure of the charging passage 533 (see FIGS.43 and 44). The pressure relief line 533 has the second end opening tothe outside to form the drain port 554 through which the hydraulic fluidfrom the relief valve 552 is drained.

[0386] Connected to the drain port 554 is the cooling conduit 591,through which the hydraulic fluid drained from the drain port 554 isreturned to the hydraulic tank, in the same manner as the ninthembodiment.

[0387] The thus arranged pump unit 500′ of this embodiment also producesthe same effects as those of the ninth embodiment.

[0388] Alternative to the conduit portion 533 b′, it is possible to formin the peripheral wall of the common housing 520 a communication holehaving a first end connected to the second end of the first bore portion533 a′ and a second end connected to the first end of the second boreportion 533 c′.

[0389] This specification is by no means intended to restrict thepresent invention to the preferred embodiments set forth therein.Various modifications to the pump unit, as described herein, may be madeby those skilled in the art without departing from the spirit and scopeof the present invention as defined in the appended claims.

1-28 (Cancelled).
 29. A pump unit comprising: a common housing; a firsthydraulic pump and a second hydraulic pump, both being of a variabledisplacement type and accommodated within said common housing; saidfirst hydraulic pump and said second hydraulic pump respectivelyincluding a first pump shaft and a second pump shaft, both of which haverotating shaft centers substantially parallel to each other, and a firstcontrol shaft and a second control shaft designed for controlling theinput/output flow rates of said first and second hydraulic pumps; andsaid first and second control shafts protruding outwardly from the sameside of said common housing.
 30. A pump unit comprising: a commonhousing; a first hydraulic pump and a second hydraulic pump, both beingof a variable displacement type and accommodated within said commonhousing; said first hydraulic pump and said second hydraulic pumprespectively including a first pump shaft and a second pump shaft, bothof which have rotating shaft centers substantially parallel to eachother, and a first control shaft and a second control shaft designed forcontrolling the input/output flow rates of said first and secondhydraulic pumps; a first neutral position return mechanism for urgingsaid first control shaft in such a direction as to return said firsthydraulic pump to a neutral position thereof; and a second neutralposition return mechanism for urging said second control shaft in such adirection as to return said second hydraulic pump to a neutral positionthereof.
 31. A pump unit according to claim 30, wherein said first andsecond neutral position return mechanisms are respectively provided withadjusting mechanisms for adjusting said first and second hydraulic pumpsrespectively to their exact neutral positions.
 32. A pump unitcomprising: a common housing; a first hydraulic pump and a secondhydraulic pump, both being of a variable displacement type andaccommodated within said common housing; said first hydraulic pump andsaid second hydraulic pump respectively including a first pump shaft anda second pump shaft, both of which have rotating shaft centerssubstantially parallel to each other, and a first control shaft and asecond control shaft designed for controlling the input/output flowrates of said first and second hydraulic pumps; a first neutral positionreturn mechanism for urging said first control shaft in such a directionas to return said first hydraulic pump to a neutral position thereof; asecond neutral position return mechanism for urging said second controlshaft in such a direction as to return said second hydraulic pump to aneutral position thereof; said first and second control shaftsprotruding outwardly from the same side of said common housing; and saidfirst and second neutral position return mechanisms mounted on said sameside of the common housing.
 33. A pump unit according to claim 32,wherein said first and second neutral position return mechanisms arerespectively provided with adjusting mechanisms for adjusting said firstand second hydraulic pumps respectively to their exact neutralpositions.
 34. A pump unit according to claim 32, wherein said first andsecond neutral position return mechanisms are disposed on a common baseplate detachably mounted on said same side of the common housing.